diff --git a/phpstan.neon b/phpstan.neon
index 0be50305..aecda91a 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -29,7 +29,7 @@ parameters:
     paths:
         - plugins
 
-    level: 2
+    level: 3
 
     scanDirectories:
         - program/include
diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index d286eb6b..1fff88f8 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -1036,7 +1036,7 @@ $("#rcmfd_new_category").keypress(function(event) {
 
                     // let the UI generate HTML and CSS representation for this calendar
                     $html = $this->ui->calendar_list_item($id, $prop, $jsenv);
-                    $cal  = $jsenv[$id];
+                    $cal  = $jsenv[$id] ?? [];
                     $cal['editname'] = $editname;
                     $cal['html']     = $html;
 
diff --git a/plugins/calendar/drivers/caldav/caldav_calendar.php b/plugins/calendar/drivers/caldav/caldav_calendar.php
index a4180020..165fd460 100644
--- a/plugins/calendar/drivers/caldav/caldav_calendar.php
+++ b/plugins/calendar/drivers/caldav/caldav_calendar.php
@@ -450,7 +450,7 @@ class caldav_calendar extends kolab_storage_dav_folder
 
         $this->events = [$event['uid'] => $this->_to_driver_event($object, true)];
 
-        return true;
+        return $saved;
     }
 
     /**
diff --git a/plugins/calendar/drivers/caldav/caldav_driver.php b/plugins/calendar/drivers/caldav/caldav_driver.php
index c0e5d250..85727b0b 100644
--- a/plugins/calendar/drivers/caldav/caldav_driver.php
+++ b/plugins/calendar/drivers/caldav/caldav_driver.php
@@ -99,6 +99,8 @@ class caldav_driver extends kolab_driver
 
     /**
      * Convert kolab_storage_folder into caldav_calendar
+     *
+     * @return caldav_calendar|kolab_user_calendar
      */
     protected function _to_calendar($folder)
     {
@@ -278,7 +280,7 @@ class caldav_driver extends kolab_driver
      *
      * @param string $id Calendar identifier
      *
-     * @return ?caldav_calendar Object nor null if calendar doesn't exist
+     * @return caldav_calendar|caldav_invitation_calendar|null Object nor null if calendar doesn't exist
      */
     public function get_calendar($id)
     {
diff --git a/plugins/calendar/drivers/calendar_driver.php b/plugins/calendar/drivers/calendar_driver.php
index 42e3cbd4..f493fc41 100644
--- a/plugins/calendar/drivers/calendar_driver.php
+++ b/plugins/calendar/drivers/calendar_driver.php
@@ -296,7 +296,7 @@ abstract class calendar_driver
      *                     See FILTER_* constants for possible values.
      * @param bool  $full  If true, recurrence exceptions shall be added
      *
-     * @return array Event object as hash array
+     * @return ?array Event object as hash array
      */
     abstract public function get_event($event, $scope = 0, $full = false);
 
@@ -420,11 +420,11 @@ abstract class calendar_driver
      *         id: Event identifier
      *   calendar: Calendar identifier
      *
-     * @return string Attachment body
+     * @return string|false Attachment body
      */
     public function get_attachment_body($id, $event)
     {
-        return '';
+        return false;
     }
 
     /**
@@ -434,7 +434,7 @@ abstract class calendar_driver
      *                         or an URI from a stored link referencing a mail message.
      * @param string $folder  IMAP folder the message resides in
      *
-     * @return array An struct referencing the given IMAP message
+     * @return array|false An struct referencing the given IMAP message
      */
     public function get_message_reference($uri_or_headers, $folder = null)
     {
@@ -480,7 +480,7 @@ abstract class calendar_driver
      * @param int    $start Requested period start date/time as unix timestamp
      * @param int    $end   Requested period end date/time as unix timestamp
      *
-     * @return array List of busy timeslots within the requested range
+     * @return array|false List of busy timeslots within the requested range
      */
     public function get_freebusy_list($email, $start, $end)
     {
@@ -551,7 +551,7 @@ abstract class calendar_driver
      *         id: Event identifier
      *   calendar: Calendar identifier
      *
-     * @return array List of changes, each as a hash array:
+     * @return array|false List of changes, each as a hash array:
      *         rev: Revision number
      *        type: Type of the change (create, update, move, delete)
      *        date: Change date
@@ -573,7 +573,7 @@ abstract class calendar_driver
      * @param mixed $rev1 Old Revision
      * @param mixed $rev2 New Revision
      *
-     * @return array List of property changes, each as a hash array:
+     * @return array|false List of property changes, each as a hash array:
      *    property: Revision number
      *         old: Old property value
      *         new: Updated property value
@@ -591,7 +591,7 @@ abstract class calendar_driver
      *  calendar: Calendar identifier
      * @param mixed  $rev Revision number
      *
-     * @return array Event object as hash array
+     * @return array|false Event object as hash array
      * @see self::get_event()
      */
     public function get_event_revison($event, $rev)
@@ -608,7 +608,7 @@ abstract class calendar_driver
      *  calendar: Calendar identifier
      * @param mixed  $rev Revision number
      *
-     * @return boolean True on success, False on failure
+     * @return bool True on success, False on failure
      */
     public function restore_event_revision($event, $rev)
     {
@@ -794,7 +794,7 @@ abstract class calendar_driver
     public static function parse_contact($contact, $source)
     {
         if (!is_array($contact)) {
-            return;
+            return null;
         }
 
         if (!empty($contact['birthday']) && is_array($contact['birthday'])) {
@@ -802,7 +802,7 @@ abstract class calendar_driver
         }
 
         if (empty($contact['birthday'])) {
-            return;
+            return null;
         }
 
         try {
@@ -818,7 +818,7 @@ abstract class calendar_driver
                 true,
                 false
             );
-            return;
+            return null;
         }
 
         $rcmail       = rcmail::get_instance();
diff --git a/plugins/calendar/drivers/database/database_driver.php b/plugins/calendar/drivers/database/database_driver.php
index 15fe3924..db7d5cc5 100644
--- a/plugins/calendar/drivers/database/database_driver.php
+++ b/plugins/calendar/drivers/database/database_driver.php
@@ -1017,7 +1017,7 @@ class database_driver extends calendar_driver
      * @param int   $scope Bitmask defining the scope to search events in
      * @param bool  $full  If true, recurrence exceptions shall be added
      *
-     * @return array Hash array with event properties
+     * @return ?array Hash array with event properties
      */
     public function get_event($event, $scope = 0, $full = false)
     {
@@ -1072,7 +1072,7 @@ class database_driver extends calendar_driver
             return $this->cache[$id];
         }
 
-        return false;
+        return null;
     }
 
     /**
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 2114d494..d7c45094 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -352,7 +352,7 @@ class kolab_driver extends calendar_driver
      *
      * @param string $id Calendar identifier (encoded imap folder name)
      *
-     * @return kolab_calendar Object nor null if calendar doesn't exist
+     * @return kolab_calendar|kolab_invitation_calendar|null Object nor null if calendar doesn't exist
      */
     public function get_calendar($id)
     {
@@ -585,7 +585,7 @@ class kolab_driver extends calendar_driver
      * Fetch a single event
      *
      * @see calendar_driver::get_event()
-     * @return array Hash array with event properties, false if not found
+     * @return ?array Hash array with event properties, false if not found
      */
     public function get_event($event, $scope = 0, $full = false)
     {
@@ -621,7 +621,7 @@ class kolab_driver extends calendar_driver
             }
         }
 
-        return false;
+        return null;
     }
 
     /**
@@ -1843,7 +1843,7 @@ class kolab_driver extends calendar_driver
     public function list_attachments($event)
     {
         if (!($storage = $this->get_calendar($event['calendar']))) {
-            return false;
+            return [];
         }
 
         $event = $storage->get_event($event['id']);
@@ -2260,7 +2260,7 @@ class kolab_driver extends calendar_driver
      *
      * @param array $event Hash array with event properties
      *
-     * @return array List of changes, each as a hash array
+     * @return array|false List of changes, each as a hash array
      * @see calendar_driver::get_event_changelog()
      */
     public function get_event_changelog($event)
@@ -2286,7 +2286,7 @@ class kolab_driver extends calendar_driver
      * @param mixed $rev1  Old Revision
      * @param mixed $rev2  New Revision
      *
-     * @return array List of property changes, each as a hash array
+     * @return array|false List of property changes, each as a hash array
      * @see calendar_driver::get_event_diff()
      */
     public function get_event_diff($event, $rev1, $rev2)
@@ -2413,7 +2413,7 @@ class kolab_driver extends calendar_driver
      * @param array $event Hash array with event properties
      * @param mixed $rev   Revision number
      *
-     * @return array Event object as hash array
+     * @return array|false Event object as hash array
      * @see calendar_driver::get_event_revison()
      */
     public function get_event_revison($event, $rev, $internal = false)
diff --git a/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php b/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
index 3cdb39b6..b89f772f 100644
--- a/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_invitation_calendar.php
@@ -118,7 +118,7 @@ class kolab_invitation_calendar
     /**
      * Getter for the Cyrus mailbox identifier corresponding to this folder
      *
-     * @return string Mailbox ID
+     * @return ?string Mailbox ID
      */
     public function get_mailbox_id()
     {
diff --git a/plugins/calendar/drivers/ldap/resources_driver_ldap.php b/plugins/calendar/drivers/ldap/resources_driver_ldap.php
index c12067fc..c077e451 100644
--- a/plugins/calendar/drivers/ldap/resources_driver_ldap.php
+++ b/plugins/calendar/drivers/ldap/resources_driver_ldap.php
@@ -63,7 +63,7 @@ class resources_driver_ldap extends resources_driver
             $results = $ldap->list_records();
         }
 
-        if ($results instanceof ArrayAccess) {
+        if ($results instanceof Iterator) {
             foreach ($results as $i => $rec) {
                 $results[$i] = $this->decode_resource($rec);
             }
diff --git a/plugins/calendar/drivers/resources_driver.php b/plugins/calendar/drivers/resources_driver.php
index b7a779bc..22c49c96 100644
--- a/plugins/calendar/drivers/resources_driver.php
+++ b/plugins/calendar/drivers/resources_driver.php
@@ -59,7 +59,7 @@ abstract class resources_driver
      *
      * @param string $id Owner identifier
      *
-     * @return array Resource object as hash array
+     * @return ?array Resource owner object as hash array
      */
     public function get_resource_owner($id)
     {
diff --git a/plugins/calendar/lib/calendar_itip.php b/plugins/calendar/lib/calendar_itip.php
index a1cef132..a50ef06f 100644
--- a/plugins/calendar/lib/calendar_itip.php
+++ b/plugins/calendar/lib/calendar_itip.php
@@ -142,7 +142,7 @@ class calendar_itip extends libcalendaring_itip
      * @param array  $event    Hash array with event properties
      * @param string $attendee Attendee email address
      *
-     * @return string Invitation token
+     * @return string|false Invitation token
      */
     public function store_invitation($event, $attendee)
     {
diff --git a/plugins/kolab_2fa/kolab_2fa.php b/plugins/kolab_2fa/kolab_2fa.php
index 69cfd50d..97ce06ae 100644
--- a/plugins/kolab_2fa/kolab_2fa.php
+++ b/plugins/kolab_2fa/kolab_2fa.php
@@ -336,7 +336,7 @@ class kolab_2fa extends rcube_plugin
      * Load driver class for the given authentication factor
      *
      * @param string $factor Factor identifier (<method>:<id>)
-     * @return Kolab2FA\Driver\Base
+     * @return Kolab2FA\Driver\Base|false
      */
     public function get_driver($factor)
     {
diff --git a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
index aea1db11..a2dbbad5 100644
--- a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
+++ b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
@@ -290,7 +290,7 @@ class carddav_contacts extends rcube_addressbook
      * @param int   $subset  Only return this number of records, use negative values for tail
      * @param bool  $nocount True to skip the count query (select only)
      *
-     * @return array Indexed list of contact records, each a hash array
+     * @return rcube_result_set Indexed list of contact records, each a hash array
      */
     public function list_records($cols = null, $subset = 0, $nocount = false)
     {
@@ -1004,7 +1004,7 @@ class carddav_contacts extends rcube_addressbook
      * @param string $gid Group identifier
      * @param array  $ids List of contact identifiers to be removed
      *
-     * @return bool
+     * @return int Numer of removed contacts
      */
     public function remove_from_group($gid, $ids)
     {
@@ -1013,7 +1013,7 @@ class carddav_contacts extends rcube_addressbook
         $list = $this->distlists[$gid];
 
         if (!$list) {
-            return false;
+            return 0;
         }
 
         if (!is_array($ids)) {
@@ -1021,9 +1021,12 @@ class carddav_contacts extends rcube_addressbook
         }
 
         $new_member = [];
+        $removed = 0;
         foreach ((array) $list['member'] as $member) {
             if (!in_array($member['ID'], $ids)) {
                 $new_member[] = $member;
+            } else {
+                $removed++;
             }
         }
 
@@ -1040,19 +1043,19 @@ class carddav_contacts extends rcube_addressbook
                 true,
                 false
             );
-        } else {
-            // remove group assigments in local cache
-            foreach ($ids as $id) {
-                $j = array_search($gid, $this->groupmembers[$id]);
-                unset($this->groupmembers[$id][$j]);
-            }
 
-            $this->distlists[$gid] = $list;
-
-            return true;
+            return 0;
         }
 
-        return false;
+        // remove group assigments in local cache
+        foreach ($ids as $id) {
+            $j = array_search($gid, $this->groupmembers[$id]);
+            unset($this->groupmembers[$id][$j]);
+        }
+
+        $this->distlists[$gid] = $list;
+
+        return $removed;
     }
 
     /**
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
index f062a118..656556bc 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
@@ -344,7 +344,7 @@ class kolab_contacts extends rcube_addressbook
      * @param int   $subset  Only return this number of records, use negative values for tail
      * @param bool  $nocount True to skip the count query (select only)
      *
-     * @return array Indexed list of contact records, each a hash array
+     * @return rcube_result_set Indexed list of contact records, each a hash array
      */
     public function list_records($cols = null, $subset = 0, $nocount = false)
     {
@@ -877,6 +877,7 @@ class kolab_contacts extends rcube_addressbook
             );
             return false;
         } else {
+            // @phpstan-ignore-next-line FIXME: this one might be valid error
             $id = $this->uid2id($list['uid']);
             $this->distlists[$id] = $list;
             $result = ['id' => $id, 'name' => $name];
@@ -924,7 +925,7 @@ class kolab_contacts extends rcube_addressbook
      * @param string $newname New name to set for this group
      * @param string $newid   New group identifier (if changed, otherwise don't set)
      *
-     * @return bool New name on success, false if no data was changed
+     * @return string|bool New name on success, false if no data was changed
      */
     public function rename_group($gid, $newname, &$newid)
     {
@@ -1019,7 +1020,7 @@ class kolab_contacts extends rcube_addressbook
                 false
             );
 
-            $added = false;
+            $added = 0;
             $this->set_error(self::ERROR_SAVING, 'errorsaving');
         } else {
             $this->distlists[$gid] = $list;
@@ -1044,13 +1045,17 @@ class kolab_contacts extends rcube_addressbook
 
         $this->_fetch_groups();
         if (!($list = $this->distlists[$gid])) {
-            return false;
+            return 0;
         }
 
         $new_member = [];
+        $removed = 0;
+
         foreach ((array)$list['member'] as $member) {
             if (!in_array($member['ID'], $ids)) {
                 $new_member[] = $member;
+            } else {
+                $removed++;
             }
         }
 
@@ -1067,17 +1072,19 @@ class kolab_contacts extends rcube_addressbook
                 true,
                 false
             );
-        } else {
-            // remove group assigments in local cache
-            foreach ($ids as $id) {
-                $j = array_search($gid, $this->groupmembers[$id]);
-                unset($this->groupmembers[$id][$j]);
-            }
-            $this->distlists[$gid] = $list;
-            return true;
+
+            return 0;
         }
 
-        return false;
+        // remove group assigments in local cache
+        foreach ($ids as $id) {
+            $j = array_search($gid, $this->groupmembers[$id]);
+            unset($this->groupmembers[$id][$j]);
+        }
+
+        $this->distlists[$gid] = $list;
+
+        return $removed;
     }
 
     /**
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
index ad5e8c24..980b8527 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
@@ -97,7 +97,7 @@ class kolab_contacts_driver
         $folder   = kolab_storage::get_folder($folderId);
 
         if ($folder && kolab_storage::folder_delete($folder->name)) {
-            return $folderId;
+            return true;
         }
 
         return false;
diff --git a/plugins/kolab_addressbook/kolab_addressbook.php b/plugins/kolab_addressbook/kolab_addressbook.php
index 61fd84f7..4d695aec 100644
--- a/plugins/kolab_addressbook/kolab_addressbook.php
+++ b/plugins/kolab_addressbook/kolab_addressbook.php
@@ -1022,7 +1022,7 @@ class kolab_addressbook extends rcube_plugin
     {
         $source = trim(rcube_utils::get_input_value('_source', rcube_utils::INPUT_GPC, true));
 
-        if ($source && ($result = $this->driver->folder_delete($source))) {
+        if ($source && $this->driver->folder_delete($source)) {
             $storage   = $this->rc->get_storage();
             $delimiter = $storage->get_hierarchy_delimiter();
 
diff --git a/plugins/kolab_chat/drivers/mattermost.php b/plugins/kolab_chat/drivers/mattermost.php
index c081d002..8bac1f1d 100644
--- a/plugins/kolab_chat/drivers/mattermost.php
+++ b/plugins/kolab_chat/drivers/mattermost.php
@@ -266,7 +266,7 @@ class kolab_chat_mattermost
     /**
      * Returns the Mattermost channels list for the user
      *
-     * @return array Channels list (id, name, display_name, etc.)
+     * @return array|false Channels list (id, name, display_name, etc.)
      */
     protected function get_channels_list()
     {
diff --git a/plugins/kolab_delegation/kolab_delegation_engine.php b/plugins/kolab_delegation/kolab_delegation_engine.php
index 8d12d845..0684b593 100644
--- a/plugins/kolab_delegation/kolab_delegation_engine.php
+++ b/plugins/kolab_delegation/kolab_delegation_engine.php
@@ -449,11 +449,11 @@ class kolab_delegation_engine
 
             $user = $this->parse_ldap_record($record);
 
-            if ($user['uid']) {
+            if ($uid = $user['uid']) {
                 $display = rcube_addressbook::compose_search_name($record);
-                $user    = ['name' => $user['uid'], 'display' => $display];
+                $user    = ['name' => $uid, 'display' => $display];
                 $users[] = $user;
-                $keys[]  = $display ?: $user['uid'];
+                $keys[]  = $display ?: $uid;
             }
         }
 
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index 447446ba..a79e9ee8 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -578,7 +578,7 @@ class kolab_folders extends rcube_plugin
      *
      * @param string $type Folder type
      *
-     * @return string Folder name
+     * @return ?string Folder name
      */
     public function get_default_folder($type)
     {
diff --git a/plugins/kolab_notes/kolab_notes.php b/plugins/kolab_notes/kolab_notes.php
index 4a09a49c..3285bc56 100644
--- a/plugins/kolab_notes/kolab_notes.php
+++ b/plugins/kolab_notes/kolab_notes.php
@@ -892,7 +892,7 @@ class kolab_notes extends rcube_plugin
      *
      * @param array $note Hash array with note properties
      *
-     * @return array List of changes, each as a hash array
+     * @return array|false List of changes, each as a hash array
      */
     public function get_changelog($note)
     {
@@ -916,7 +916,7 @@ class kolab_notes extends rcube_plugin
      * @param mixed $note UID string or hash array with note properties
      * @param mixed $rev Revision number
      *
-     * @return array Note object as hash array
+     * @return array|false Note object as hash array
      */
     public function get_revison($note, $rev)
     {
@@ -949,7 +949,7 @@ class kolab_notes extends rcube_plugin
      * @param mixed $rev1   Revision: "from"
      * @param mixed $rev2   Revision: "to"
      *
-     * @return array List of property changes, each as a hash array
+     * @return array|false List of property changes, each as a hash array
      */
     public function get_diff($note, $rev1, $rev2)
     {
@@ -1007,10 +1007,10 @@ class kolab_notes extends rcube_plugin
      * Command the backend to restore a certain revision of a note.
      * This shall replace the current object with an older version.
      *
-     * @param array  $note Hash array with note properties (id, list)
-     * @param mixed  $rev Revision number
+     * @param array $note Hash array with note properties (id, list)
+     * @param mixed $rev  Revision number
      *
-     * @return boolean True on success, False on failure
+     * @return bool True on success, False on failure
      */
     public function restore_revision($note, $rev)
     {
@@ -1152,7 +1152,7 @@ class kolab_notes extends rcube_plugin
 
                     // let the UI generate HTML and CSS representation for this calendar
                     $html = $this->ui->folder_list_item($id, $prop, $jsenv, true);
-                    $prop += (array)$jsenv[$id];
+                    $prop += $jsenv[$id] ?? [];
                     $prop['editname'] = $editname;
                     $prop['html'] = $html;
 
diff --git a/plugins/libcalendaring/lib/libcalendaring_itip.php b/plugins/libcalendaring/lib/libcalendaring_itip.php
index f43e319c..9f9ac578 100644
--- a/plugins/libcalendaring/lib/libcalendaring_itip.php
+++ b/plugins/libcalendaring/lib/libcalendaring_itip.php
@@ -958,14 +958,13 @@ class libcalendaring_itip
         return $table->show();
     }
 
-
     /**
      * Create iTIP invitation token for later replies via URL
      *
      * @param array  $event    Hash array with event properties
      * @param string $attendee Attendee email address
      *
-     * @return string Invitation token
+     * @return string|false Invitation token
      */
     public function store_invitation($event, $attendee)
     {
@@ -981,7 +980,6 @@ class libcalendaring_itip
     public function cancel_itip_invitation($event)
     {
         // empty stub
-        return false;
     }
 
     /**
diff --git a/plugins/libcalendaring/lib/libcalendaring_recurrence.php b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
index f396a2af..3b06bd01 100644
--- a/plugins/libcalendaring/lib/libcalendaring_recurrence.php
+++ b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
@@ -153,7 +153,7 @@ class libcalendaring_recurrence
     {
         // recurrence end date is given
         if (isset($this->recurrence['UNTIL']) && $this->recurrence['UNTIL'] instanceof DateTimeInterface) {
-            return $this->recurrence['UNTIL'];
+            return $this->toDateTime($this->recurrence['UNTIL']);
         }
 
         // Run through all items till we reach the end, or limit of iterations
@@ -165,12 +165,13 @@ class libcalendaring_recurrence
         } catch (Exception $e) {
             // do nothing
         }
+
         /*
-                if (empty($end) && isset($this->event['start']) && $this->event['start'] instanceof DateTimeInterface) {
-                    // determine a reasonable end date if none given
-                    $end = clone $this->event['start'];
-                    $end->add(new DateInterval('P100Y'));
-                }
+        if (empty($end) && isset($this->event['start']) && $this->event['start'] instanceof DateTimeInterface) {
+            // determine a reasonable end date if none given
+            $end = clone $this->event['start'];
+            $end->add(new DateInterval('P100Y'));
+        }
         */
 
         return isset($end) ? $this->toDateTime($end) : false;
diff --git a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
index b27a762b..41b2ed3a 100644
--- a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
+++ b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
@@ -314,7 +314,7 @@ class libcalendaring_vcalendar implements Iterator
     /**
      * Import objects from an already parsed Sabre\VObject\Component object
      *
-     * @param Sabre\VObject\Component $vobject Component to read from
+     * @param VObject\Component $vobject Component to read from
      *
      * @return array List of events extracted from the file
      */
@@ -399,7 +399,7 @@ class libcalendaring_vcalendar implements Iterator
     /**
      * Convert the given VEvent object to a libkolab compatible array representation
      *
-     * @param Sabre\VObject\Component\VEvent|Sabre\VObject\Component\VTodo $ve VEvent object to convert
+     * @param VObject\Component\VEvent|VObject\Component\VTodo $ve VEvent object to convert
      *
      * @return array Hash array with object properties
      */
@@ -937,7 +937,7 @@ class libcalendaring_vcalendar implements Iterator
      * @param bool             $utc      Set as UTC date
      * @param bool             $dateonly Set as VALUE=DATE property
      *
-     * @return Sabre\VObject\Property
+     * @return VObject\Property
      */
     public function datetime_prop($cal, $name, $dt, $utc = false, $dateonly = null, $set_type = false)
     {
@@ -1002,7 +1002,6 @@ class libcalendaring_vcalendar implements Iterator
         return $params;
     }
 
-
     /**
      * Export events to iCalendar format
      *
@@ -1012,7 +1011,7 @@ class libcalendaring_vcalendar implements Iterator
      * @param ?callable $get_attachment Optional callback function to fetch attachment contents
      * @param bool      $with_timezones Add VTIMEZONE block with timezone definitions for the included events
      *
-     * @return string Events in iCalendar format (http://tools.ietf.org/html/rfc5545)
+     * @return string|true Events in iCalendar format (http://tools.ietf.org/html/rfc5545)
      */
     public function export($objects, $method = null, $write = false, $get_attachment = null, $with_timezones = true)
     {
@@ -1056,9 +1055,9 @@ class libcalendaring_vcalendar implements Iterator
         if ($write) {
             echo "END:VCALENDAR\r\n";
             return true;
-        } else {
-            return $vcal->serialize();
         }
+
+        return $vcal->serialize();
     }
 
     /**
@@ -1067,7 +1066,7 @@ class libcalendaring_vcalendar implements Iterator
      * @param array     $object         Event
      * @param ?callable $get_attachment Optional callback function to fetch attachment contents
      *
-     * @return Sabre\VObject\Component\VEvent Sabre component
+     * @return VObject\Component Sabre component
      */
     public function toSabreComponent($object, $get_attachment = null)
     {
@@ -1416,10 +1415,10 @@ class libcalendaring_vcalendar implements Iterator
      * @param string $tzid Timezone ID as used in PHP's Date functions
      * @param int    $from Unix timestamp with first date/time in this timezone
      * @param int    $to   Unix timestap with last date/time in this timezone
-     * @param Sabre\VObject\Component\VCalendar $cal Optional VCalendar component
+     * @param VObject\Component\VCalendar $cal Optional VCalendar component
      *
-     * @return Sabre\VObject\Component|false Object representing a VTIMEZONE definition
-     *                                       or false if no timezone information is available
+     * @return VObject\Component|false Object representing a VTIMEZONE definition
+     *                                 or false if no timezone information is available
      */
     public static function get_vtimezone($tzid, $from = 0, $to = 0, $cal = null)
     {
@@ -1552,8 +1551,6 @@ class libcalendaring_vcalendar implements Iterator
         if (empty($this->objects[$this->iteratorkey]) && $this->fp) {
             $this->_parse_next(true);
         }
-
-        return $this->valid();
     }
 
     #[\ReturnTypeWillChange]
@@ -1567,7 +1564,6 @@ class libcalendaring_vcalendar implements Iterator
     {
         return !empty($this->objects[$this->iteratorkey]);
     }
-
 }
 
 
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index 9f80367e..b2461ecc 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -720,7 +720,7 @@ class libcalendaring extends rcube_plugin
 
                 try {
                     $interval = new DateInterval(trim($alarm['trigger'], '+-'));
-                    $interval->invert = $alarm['trigger'][0] == '-';
+                    $interval->invert = $alarm['trigger'][0] == '-' ? 1 : 0;
                     $notify_time = clone $refdate;
                     $notify_time->add($interval);
                 } catch (Exception $e) {
@@ -1244,7 +1244,7 @@ class libcalendaring extends rcube_plugin
      * @param string $mime_id Message part ID and object index (e.g. '1.2:0')
      * @param string $type    Object type filter (optional)
      *
-     * @return array Hash array with the parsed iCal
+     * @return ?array Hash array with the parsed iCal
      */
     public function mail_get_itip_object($mbox, $uid, $mime_id, $type = null)
     {
diff --git a/plugins/libkolab/lib/kolab_bonnie_api_client.php b/plugins/libkolab/lib/kolab_bonnie_api_client.php
index 5942eac1..4279cc1a 100644
--- a/plugins/libkolab/lib/kolab_bonnie_api_client.php
+++ b/plugins/libkolab/lib/kolab_bonnie_api_client.php
@@ -160,9 +160,12 @@ class kolab_bonnie_api_client
     /**
      * Do the HTTP request
      *
-     * @param  string  $payload  Data to send
+     * @param array $payload Data to send
+     * @param bool  $sign    Enable signed request
+     *
+     * @return array
      */
-    protected function send_request($payload, $sign = true)
+    protected function send_request(array $payload, $sign = true)
     {
         try {
             $payload_ = json_encode($payload);
@@ -175,6 +178,7 @@ class kolab_bonnie_api_client
             }
 
             $this->_debug('REQUEST', $payload, $this->headers);
+
             $request = libkolab::http_request($this->url, 'POST', ['timeout' => $this->timeout]);
             $request->setHeader($this->headers);
             $request->setAuth($this->username, $this->password);
@@ -198,7 +202,7 @@ class kolab_bonnie_api_client
             return ['id' => $payload['id'], 'error' => $e->getMessage(), 'code' => -32000];
         }
 
-        return is_array($result) ? $result : [];
+        return isset($result) && is_array($result) ? $result : [];
     }
 
     /**
diff --git a/plugins/libkolab/lib/kolab_date_recurrence.php b/plugins/libkolab/lib/kolab_date_recurrence.php
index 202328fb..d64f4250 100644
--- a/plugins/libkolab/lib/kolab_date_recurrence.php
+++ b/plugins/libkolab/lib/kolab_date_recurrence.php
@@ -133,7 +133,7 @@ class kolab_date_recurrence
     /**
      * Get the end date of the last occurence of this recurrence cycle
      *
-     * @return DateTime|bool End datetime of the last event or False if recurrence exceeds limit
+     * @return DateTimeInterface|bool End datetime of the last event or False if recurrence exceeds limit
      */
     public function end()
     {
diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index 629335e6..2d9088be 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -283,7 +283,7 @@ abstract class kolab_format
      * @param cDateTime    $cdt     The libkolabxml datetime object
      * @param DateTimeZone $dest_tz The timezone to convert the date to
      *
-     * @return libcalendaring_datetime PHP datetime instance
+     * @return libcalendaring_datetime|null PHP datetime instance, Null on invalid input
      */
     public static function php_datetime($cdt, $dest_tz = null)
     {
@@ -463,7 +463,7 @@ abstract class kolab_format
      *
      * @param float $v Version value to convert
      *
-     * @return int Constant value of either kolabobject::KolabV2 or kolabobject::KolabV3 or false if kolabobject module isn't available
+     * @return int|false Constant value of either kolabobject::KolabV2 or kolabobject::KolabV3 or false if kolabobject module isn't available
      */
     protected function libversion($v = null)
     {
diff --git a/plugins/libkolab/lib/kolab_ldap.php b/plugins/libkolab/lib/kolab_ldap.php
index 08a46c29..0ce9e408 100644
--- a/plugins/libkolab/lib/kolab_ldap.php
+++ b/plugins/libkolab/lib/kolab_ldap.php
@@ -301,12 +301,12 @@ class kolab_ldap extends rcube_ldap_generic
      *
      * @param string $dn DN
      *
-     * @return array Record data
+     * @return array|null Record data
      */
     public function get_record($dn)
     {
         if (!$this->ready) {
-            return;
+            return null;
         }
 
         if ($rec = $this->get_entry($dn, $this->attributes)) {
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index a84c101c..8c0c0ddb 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -190,7 +190,7 @@ class kolab_storage
      *
      * @param string $type Data type to list folders for (contact,distribution-list,event,task,note)
      *
-     * @return object kolab_storage_folder  The folder object
+     * @return kolab_storage_folder|null The folder object
      */
     public static function get_default_folder($type)
     {
@@ -209,7 +209,7 @@ class kolab_storage
      * @param string $folder IMAP folder to access (UTF7-IMAP)
      * @param string $type   Expected folder type
      *
-     * @return object kolab_storage_folder  The folder object
+     * @return kolab_storage_folder|null The folder object
      */
     public static function get_folder($folder, $type = null)
     {
@@ -223,7 +223,7 @@ class kolab_storage
      * @param string $uid  Object UID
      * @param string $type Object type (contact,event,task,journal,file,note,configuration)
      *
-     * @return array The Kolab object represented as hash array or false if not found
+     * @return array|false The Kolab object represented as hash array or false if not found
      */
     public static function get_object($uid, $type)
     {
@@ -864,7 +864,7 @@ class kolab_storage
     public static function list_folders($root = '', $mbox = '*', $filter = null, $subscribed = null, &$folderdata = [])
     {
         if (!self::setup()) {
-            return null;
+            return [];
         }
 
         // use IMAP subscriptions
@@ -1192,7 +1192,7 @@ class kolab_storage
      *
      * @param string $folder Folder name (UTF7-IMAP)
      *
-     * @return string Folder type
+     * @return string|null Folder type
      */
     public static function folder_type($folder)
     {
@@ -1412,12 +1412,12 @@ class kolab_storage
      * @param string $type  Folder type
      * @param array  $props Folder properties (color, etc)
      *
-     * @return string Folder name
+     * @return string|null Folder name
      */
     public static function create_default_folder($type, $props = [])
     {
         if (!self::setup()) {
-            return;
+            return null;
         }
 
         $folders = self::$imap->get_metadata('*', [kolab_storage::CTYPE_KEY_PRIVATE]);
@@ -1442,7 +1442,7 @@ class kolab_storage
             }
 
             if (!$default_name) {
-                return;
+                return null;
             }
 
             $folder = rcube_charset::convert($default_name, RCUBE_CHARSET, 'UTF7-IMAP');
@@ -1455,7 +1455,7 @@ class kolab_storage
 
             if (!self::$imap->folder_exists($folder)) {
                 if (!self::$imap->create_folder($folder)) {
-                    return;
+                    return null;
                 }
             }
 
diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index 3d00b077..be7bf574 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -1011,7 +1011,7 @@ class kolab_storage_cache
      * @param string $type   Requested object type or * for all
      * @param string $folder IMAP folder to read from
      *
-     * @return array List of parsed Kolab objects
+     * @return kolab_storage_dataset List of parsed Kolab objects
      */
     protected function _fetch($index, $type = null, $folder = null)
     {
@@ -1109,8 +1109,8 @@ class kolab_storage_cache
     /**
      * Write records into cache using extended inserts to reduce the number of queries to be executed
      *
-     * @param int   $msguid Message UID. Set 0 to commit buffered inserts
-     * @param array $object Kolab object to cache
+     * @param int    $msguid Message UID. Set 0 to commit buffered inserts
+     * @param ?array $object Kolab object to cache
      */
     protected function _extended_insert($msguid, $object)
     {
diff --git a/plugins/libkolab/lib/kolab_storage_config.php b/plugins/libkolab/lib/kolab_storage_config.php
index 4fbf5875..c1b2c31d 100644
--- a/plugins/libkolab/lib/kolab_storage_config.php
+++ b/plugins/libkolab/lib/kolab_storage_config.php
@@ -276,7 +276,7 @@ class kolab_storage_config
      *
      * @param string|array $params Object UUID or Message folder, UID, Search headers (Message-Id, Date)
      *
-     * @return string $url Member URI
+     * @return string|null $url Member URI
      */
     public static function build_member_url($params)
     {
@@ -348,7 +348,7 @@ class kolab_storage_config
      *
      * @param string $url Member URI
      *
-     * @return array Message folder, UID, Search headers (Message-Id, Date)
+     * @return array|false Message folder, UID, Search headers (Message-Id, Date)
      */
     public static function parse_member_url($url)
     {
@@ -570,12 +570,12 @@ class kolab_storage_config
      * @param array $records   List of kolab objects
      * @param bool  $no_return Don't return anything
      *
-     * @return array List of tags
+     * @return array|null List of tags
      */
     public function apply_tags(&$records, $no_return = false)
     {
         if (empty($records) && $no_return) {
-            return;
+            return null;
         }
 
         // first convert categories into tags
diff --git a/plugins/libkolab/lib/kolab_storage_dav.php b/plugins/libkolab/lib/kolab_storage_dav.php
index 24298104..6ba34a82 100644
--- a/plugins/libkolab/lib/kolab_storage_dav.php
+++ b/plugins/libkolab/lib/kolab_storage_dav.php
@@ -113,7 +113,7 @@ class kolab_storage_dav
      * @param string $uid  Object UID
      * @param string $type Object type (contact,event,task,journal,file,note,configuration)
      *
-     * @return array The Kolab object represented as hash array or false if not found
+     * @return array|false The Kolab object represented as hash array or false if not found
      */
     public function get_object($uid, $type)
     {
@@ -312,7 +312,7 @@ class kolab_storage_dav
     public function folder_selector($type, $attrs, $current = '')
     {
         // TODO
-        return null;
+        return null; // @phpstan-ignore-line
     }
 
     /**
@@ -420,7 +420,7 @@ class kolab_storage_dav
      * @param string $folder Folder name
      * @param bool   $temp   Only subscribe temporarily for the current session
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function folder_subscribe($folder, $temp = false)
     {
@@ -434,7 +434,7 @@ class kolab_storage_dav
      * @param string $folder Folder name
      * @param bool   $temp   Only remove temporary subscription
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function folder_unsubscribe($folder, $temp = false)
     {
@@ -459,7 +459,7 @@ class kolab_storage_dav
      *
      * @param string $folder Folder name
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function folder_activate($folder)
     {
@@ -471,7 +471,7 @@ class kolab_storage_dav
      *
      * @param string $folder Folder name
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function folder_deactivate($folder)
     {
diff --git a/plugins/libkolab/lib/kolab_storage_dav_cache.php b/plugins/libkolab/lib/kolab_storage_dav_cache.php
index 4c346cf2..5d0d192c 100644
--- a/plugins/libkolab/lib/kolab_storage_dav_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_dav_cache.php
@@ -174,12 +174,12 @@ class kolab_storage_dav_cache extends kolab_storage_cache
                 foreach ($objects as $dav_object) {
                     if ($object = $this->folder->from_dav($dav_object)) {
                         $object['_raw'] = $dav_object['data'];
-                        $this->_extended_insert(false, $object);
+                        $this->_extended_insert(0, $object);
                         unset($object['_raw']);
                     }
                 }
 
-                $this->_extended_insert(true, null);
+                $this->_extended_insert(1, null);
 
                 // check time limit and abort sync if running too long
                 if (++$i % 25 == 0 && time() - $this->sync_start > $time_limit) {
@@ -328,7 +328,6 @@ class kolab_storage_dav_cache extends kolab_storage_cache
      *
      * @param mixed  $object Hash array with object properties to save or false to delete the cache entry
      * @param string $olduid Optional old message UID (for update)
-     * @param string $unused Unused (kept for compat. with the parent class)
      */
     public function save($object, $olduid = null, $unused = null)
     {
@@ -378,10 +377,7 @@ class kolab_storage_dav_cache extends kolab_storage_cache
     /**
      * Move an existing cache entry to a new resource
      *
-     * @param string               $uid     Entry's UID
-     * @param kolab_storage_folder $target  Target storage folder instance
-     * @param string               $unused1 Unused (kept for compat. with the parent class)
-     * @param string               $unused2 Unused (kept for compat. with the parent class)
+     * @param string $uid Entry's UID
      */
     public function move($uid, $target, $unused1 = null, $unused2 = null)
     {
@@ -449,7 +445,7 @@ class kolab_storage_dav_cache extends kolab_storage_cache
      *
      * @param array $query Pseudo-SQL query as list of filter parameter triplets
      *
-     * @return int The number of objects of the given type
+     * @return int|null The number of objects of the given type, Null on error
      */
     public function count($query = [])
     {
@@ -501,8 +497,8 @@ class kolab_storage_dav_cache extends kolab_storage_cache
     /**
      * Write records into cache using extended inserts to reduce the number of queries to be executed
      *
-     * @param bool  $force  Set to false to commit buffered insert, true to force an insert
-     * @param array $object Kolab object to cache
+     * @param int    $force  Set to 0 to commit buffered insert, 1 to force an insert
+     * @param ?array $object Kolab object to cache
      */
     protected function _extended_insert($force, $object)
     {
diff --git a/plugins/libkolab/lib/kolab_storage_dav_folder.php b/plugins/libkolab/lib/kolab_storage_dav_folder.php
index 227a7f86..85f39b7c 100644
--- a/plugins/libkolab/lib/kolab_storage_dav_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_dav_folder.php
@@ -200,7 +200,7 @@ class kolab_storage_dav_folder extends kolab_storage_folder
     /**
      * Helper method to extract folder UID
      *
-     * @return string Folder's UID
+     * @return string|null Folder's UID
      */
     public function get_uid()
     {
@@ -244,7 +244,7 @@ class kolab_storage_dav_folder extends kolab_storage_folder
      *
      * @param bool $subscribed The desired subscription status: true = subscribed, false = not subscribed
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function subscribe($subscribed)
     {
@@ -723,6 +723,8 @@ class kolab_storage_dav_folder extends kolab_storage_folder
      */
     public function get_attachment($id, $event, $unused1 = null, $unused2 = false, $unused3 = null, $unused4 = false)
     {
+        /** @var array $event Overwrite type from the parent class */
+
         // Note: 'attachments' is defined when saving the data into the DAV server
         //       '_attachments' is defined after fetching the object from the DAV server
         if (is_int($id) && isset($event['attachments'][$id])) {
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index 34efb1c7..924429fe 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -148,7 +148,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
     /**
      * Helper method to extract folder UID metadata
      *
-     * @return string Folder's UID
+     * @return string|null Folder's UID
      */
     public function get_uid()
     {
@@ -216,7 +216,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
      *
      * @param bool $active The desired subscription status: true = active, false = not active
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function activate($active)
     {
@@ -238,7 +238,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
      *
      * @param bool $subscribed The desired subscription status: true = subscribed, false = not subscribed
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function subscribe($subscribed)
     {
@@ -407,9 +407,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
             if (substr($part, 0, 2) == 'i:') {
                 // attachment data is stored in XML
                 if ($object = $this->cache->get($msguid)) {
+                    $object['_attachments'] = [];
+
                     // load data from XML (attachment content is not stored in cache)
-                    if ($object['_formatobj'] && isset($object['_size'])) {
-                        $object['_attachments'] = [];
+                    if (isset($object['_formatobj']) && isset($object['_size'])) {
                         $object['_formatobj']->get_attachments($object);
                     }
 
diff --git a/plugins/libkolab/lib/kolab_storage_folder_api.php b/plugins/libkolab/lib/kolab_storage_folder_api.php
index 42b50917..cfc65710 100644
--- a/plugins/libkolab/lib/kolab_storage_folder_api.php
+++ b/plugins/libkolab/lib/kolab_storage_folder_api.php
@@ -323,12 +323,12 @@ abstract class kolab_storage_folder_api
     /**
      * Helper method to extract folder UID metadata
      *
-     * @return string Folder's UID
+     * @return string|null Folder's UID
      */
     public function get_uid()
     {
         // To be implemented by extending classes
-        return false;
+        return null;
     }
 
     /**
@@ -346,7 +346,7 @@ abstract class kolab_storage_folder_api
      *
      * @param bool $active The desired subscription status: true = active, false = not active
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function activate($active)
     {
diff --git a/plugins/libkolab/lib/kolab_storage_folder_user.php b/plugins/libkolab/lib/kolab_storage_folder_user.php
index 57e9e039..5f29b1e3 100644
--- a/plugins/libkolab/lib/kolab_storage_folder_user.php
+++ b/plugins/libkolab/lib/kolab_storage_folder_user.php
@@ -104,7 +104,7 @@ class kolab_storage_folder_user extends kolab_storage_folder_virtual
      *
      * @param bool $subscribed The desired subscription status: true = subscribed, false = not subscribed
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public function subscribe($subscribed)
     {
diff --git a/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php b/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
index aef961c7..273fd4ad 100644
--- a/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
+++ b/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
@@ -298,7 +298,7 @@ class tasklist_caldav_driver extends tasklist_driver
      *                    - color: The color of the list
      *                    - showalarms: True if alarms are enabled
      *
-     * @return mixed ID of the new list on success, False on error
+     * @return string|false ID of the new list on success, False on error
      */
     public function create_list(&$prop)
     {
@@ -723,7 +723,7 @@ class tasklist_caldav_driver extends tasklist_driver
      * @param mixed $prop UID string or hash array with task properties
      * @param mixed $rev  Revision number
      *
-     * @return array Task object as hash array
+     * @return array|false Task object as hash array
      * @see tasklist_driver::get_task_revision()
      */
     public function get_task_revison($prop, $rev)
@@ -804,7 +804,7 @@ class tasklist_caldav_driver extends tasklist_driver
      * @param mixed $rev1 Revision: "from"
      * @param mixed $rev2 Revision: "to"
      *
-     * @return array List of property changes, each as a hash array
+     * @return array|false List of property changes, each as a hash array
      * @see tasklist_driver::get_task_diff()
      */
     public function get_task_diff($prop, $rev1, $rev2)
diff --git a/plugins/tasklist/drivers/database/tasklist_database_driver.php b/plugins/tasklist/drivers/database/tasklist_database_driver.php
index 81475956..8d244636 100644
--- a/plugins/tasklist/drivers/database/tasklist_database_driver.php
+++ b/plugins/tasklist/drivers/database/tasklist_database_driver.php
@@ -109,7 +109,7 @@ class tasklist_database_driver extends tasklist_driver
      *
      * @param array $prop Hash array with list properties
      *
-     * @return mixed ID of the new list on success, False on error
+     * @return string|false ID of the new list on success, False on error
      * @see tasklist_driver::create_list()
      */
     public function create_list(&$prop)
@@ -137,7 +137,7 @@ class tasklist_database_driver extends tasklist_driver
      *
      * @param array $prop Hash array with list properties
      *
-     * @return bool True on success, Fales on failure
+     * @return bool|string True on success, Fales on failure
      * @see tasklist_driver::edit_list()
      */
     public function edit_list(&$prop)
@@ -152,7 +152,7 @@ class tasklist_database_driver extends tasklist_driver
             $this->rc->user->ID
         );
 
-        return $this->db->affected_rows($query);
+        return $this->db->affected_rows($query) > 0;
     }
 
     /**
@@ -366,12 +366,12 @@ class tasklist_database_driver extends tasklist_driver
      * @param int   $filter Bitmask defining filter criterias.
      *                      See FILTER_* constants for possible values.
      *
-     * @return array Hash array with task properties or false if not found
+     * @return array|false Hash array with task properties or false if not found
      */
     public function get_task($prop, $filter = 0)
     {
         if (is_string($prop)) {
-            $prop['uid'] = $prop;
+            $prop = ['uid' => $prop];
         }
 
         $query_col = !empty($prop['id']) ? 'task_id' : 'uid';
diff --git a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
index e2d5bd49..1b6814e8 100644
--- a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
+++ b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
@@ -355,7 +355,7 @@ class tasklist_kolab_driver extends tasklist_driver
      *       color: The color of the list
      *  showalarms: True if alarms are enabled
      *
-     * @return mixed ID of the new list on success, False on error
+     * @return string|false ID of the new list on success, False on error
      */
     public function create_list(&$prop)
     {
@@ -436,7 +436,9 @@ class tasklist_kolab_driver extends tasklist_driver
                 $prop['_reload'] = true;
             }
 
-            return $id;
+            $list['id'] = $id;
+
+            return true;
         }
 
         return false;
@@ -709,7 +711,7 @@ class tasklist_kolab_driver extends tasklist_driver
      * @param int   $filter Bitmask defining filter criterias for folders.
      *                      See FILTER_* constants for possible values.
      *
-     * @return array Hash array with task properties or false if not found
+     * @return array|false Hash array with task properties or false if not found
      */
     public function get_task($prop, $filter = 0)
     {
@@ -734,7 +736,7 @@ class tasklist_kolab_driver extends tasklist_driver
             }
         }
 
-        return $this->tasks[$id];
+        return $this->tasks[$id] ?? false;
     }
 
     /**
@@ -788,7 +790,7 @@ class tasklist_kolab_driver extends tasklist_driver
      *
      * @param array $prop Hash array with task properties
      *
-     * @return array List of changes, each as a hash array
+     * @return array|false List of changes, each as a hash array
      * @see tasklist_driver::get_task_changelog()
      */
     public function get_task_changelog($prop)
@@ -813,7 +815,7 @@ class tasklist_kolab_driver extends tasklist_driver
      * @param mixed $prop UID string or hash array with task properties
      * @param mixed $rev  Revision number
      *
-     * @return array Task object as hash array
+     * @return array|false Task object as hash array
      * @see tasklist_driver::get_task_revision()
      */
     public function get_task_revison($prop, $rev)
@@ -892,7 +894,7 @@ class tasklist_kolab_driver extends tasklist_driver
      * @param mixed $rev1 Revision "from"
      * @param mixed $rev2 Revision "to"
      *
-     * @return array List of property changes, each as a hash array
+     * @return array|false List of property changes, each as a hash array
      * @see tasklist_driver::get_task_diff()
      */
     public function get_task_diff($prop, $rev1, $rev2)
diff --git a/plugins/tasklist/drivers/tasklist_driver.php b/plugins/tasklist/drivers/tasklist_driver.php
index c166f399..e7c42d73 100644
--- a/plugins/tasklist/drivers/tasklist_driver.php
+++ b/plugins/tasklist/drivers/tasklist_driver.php
@@ -104,7 +104,7 @@ abstract class tasklist_driver
      *       color: The color of the list
      *  showalarms: True if alarms are enabled
      *
-     * @return mixed ID of the new list on success, False on error
+     * @return string|false ID of the new list on success, False on error
      */
     abstract public function create_list(&$prop);
 
@@ -218,7 +218,7 @@ abstract class tasklist_driver
      * @param int   $filter Bitmask defining filter criterias for folders.
      *                      See FILTER_* constants for possible values.
      *
-     * @return array Hash array with task properties or false if not found
+     * @return array|false Hash array with task properties or false if not found
      */
     abstract public function get_task($prop, $filter = 0);
 
@@ -329,7 +329,7 @@ abstract class tasklist_driver
      *                                                    referencing a mail message.
      * @param string                      $folder         IMAP folder the message resides in
      *
-     * @return array An struct referencing the given IMAP message
+     * @return array|false A struct referencing the given IMAP message
      */
     public function get_message_reference($uri_or_headers, $folder = null)
     {
@@ -371,7 +371,7 @@ abstract class tasklist_driver
      *         id: Task identifier
      *       list: List identifier
      *
-     * @return array List of changes, each as a hash array:
+     * @return array|false List of changes, each as a hash array:
      *         rev: Revision number
      *        type: Type of the change (create, update, move, delete)
      *        date: Change date
@@ -393,7 +393,7 @@ abstract class tasklist_driver
      * @param mixed  $rev1   Old Revision
      * @param mixed  $rev2   New Revision
      *
-     * @return array List of property changes, each as a hash array:
+     * @return array|false List of property changes, each as a hash array:
      *    property: Revision number
      *         old: Old property value
      *         new: Updated property value
@@ -411,7 +411,7 @@ abstract class tasklist_driver
      *       list: List identifier
      * @param mixed  $rev Revision number
      *
-     * @return array Task object as hash array
+     * @return array|false Task object as hash array
      * @see self::get_task()
      */
     public function get_task_revison($task, $rev)
diff --git a/plugins/tasklist/tasklist.php b/plugins/tasklist/tasklist.php
index 5e826146..6b143fb5 100644
--- a/plugins/tasklist/tasklist.php
+++ b/plugins/tasklist/tasklist.php
@@ -1026,7 +1026,7 @@ class tasklist extends rcube_plugin
                     if (empty($list['_reload'])) {
                         $this->load_ui();
                         $list['html'] = $this->ui->tasklist_list_item($insert_id, $list, $jsenv);
-                        $list += (array)$jsenv[$insert_id];
+                        $list += $jsenv[$insert_id] ?? [];
                     }
                     $this->rc->output->command('plugin.insert_tasklist', $list);
                     $success = true;
@@ -1034,11 +1034,9 @@ class tasklist extends rcube_plugin
                 break;
 
             case 'edit':
-                if ($newid = $this->driver->edit_list($list)) {
-                    $list['oldid'] = $list['id'];
-                    $list['id'] = $newid;
+                $list['oldid'] = $list['id'];
+                if ($success = $this->driver->edit_list($list)) {
                     $this->rc->output->command('plugin.update_tasklist', $list);
-                    $success = true;
                 }
                 break;
 
@@ -1065,7 +1063,7 @@ class tasklist extends rcube_plugin
 
                     // let the UI generate HTML and CSS representation for this calendar
                     $html = $this->ui->tasklist_list_item($id, $prop, $jsenv);
-                    $prop += (array)$jsenv[$id];
+                    $prop += $jsenv[$id] ?? [];
                     $prop['editname'] = $editname;
                     $prop['html'] = $html;