diff --git a/phpstan.neon b/phpstan.neon
index aecda91a..1a5b7695 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -15,8 +15,6 @@ parameters:
         - '#Access to an undefined property (calendar|tasklist)::\$itip#'
         - '#Access to an undefined property (calendar|tasklist)::\$ical#'
         - '#unknown class (kolabformat|Address|Alarm|Contact|ContactReference|Configuration|Email|RecurrenceRule|Key|Related|Url|Snippet|Telephone)#'
-        - '#Call to an undefined static method PEAR::raiseError#'
-        #- '#Class .* constructor invoked with 0 parameters#'
         - '#class (Address|Affiliation|Alarm|Attachment|Attendee|Duration|ContactReference|Configuration|Dictionary|Email|EmailReference|DayPos|FileDriver|Key|NameComponents|SnippetCollection|RecurrenceRule|Related|Relation|EventCal|Snippet|Telephone|Url|vector.*|cDatetime|XMLObject|CustomProperty) not found#i'
         - '#Call to method .* on an unknown class (Affiliation|Attachment|Attendee|cDateTime|Dictionary|EventCal|FileDriver|NameComponents|Relation|snippet|vector.*)#'
         - '#Call to static method .* on an unknown class jqueryui#'
@@ -29,7 +27,7 @@ parameters:
     paths:
         - plugins
 
-    level: 3
+    level: 4
 
     scanDirectories:
         - program/include
diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 1fff88f8..099e3267 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] ?? []; // @phpstan-ignore-line
                     $cal['editname'] = $editname;
                     $cal['html']     = $html;
 
@@ -3339,6 +3339,7 @@ $("#rcmfd_new_category").keypress(function(event) {
         $deleted   = false;
         $dontsave  = false;
         $existing = null;
+        $event_attendee = null;
 
         if ($status == 'delegated') {
             $to = rcube_utils::get_input_value('_to', rcube_utils::INPUT_POST, true);
@@ -3440,7 +3441,7 @@ $("#rcmfd_new_category").keypress(function(event) {
                 $existing = $this->find_event($event, $mode);
 
                 // we'll create a new copy if user decided to change the calendar
-                if ($existing && $cal_id && $calendar && $calendar['id'] != $existing['calendar']) {
+                if ($existing && $cal_id && $calendar['id'] != $existing['calendar']) {
                     $existing = null;
                 }
 
@@ -3449,7 +3450,6 @@ $("#rcmfd_new_category").keypress(function(event) {
                     unset($event['free_busy']);
                 }
 
-                $event_attendee   = null;
                 $update_attendees = [];
 
                 if ($existing) {
@@ -3569,7 +3569,7 @@ $("#rcmfd_new_category").keypress(function(event) {
                     } else {
                         $error_msg = $this->gettext('newerversionexists');
                     }
-                } elseif (!$existing && ($status != 'declined' || $this->rc->config->get('kolab_invitation_calendars'))) {
+                } elseif (empty($existing) && ($status != 'declined' || $this->rc->config->get('kolab_invitation_calendars'))) {
                     if ($status == 'declined'
                         || ($event['status'] ?? '') == 'CANCELLED'
                         || ($event_attendee && ($event_attendee['role'] ?? '') == 'NON-PARTICIPANT')
diff --git a/plugins/calendar/drivers/caldav/caldav_calendar.php b/plugins/calendar/drivers/caldav/caldav_calendar.php
index 165fd460..ff19b4fd 100644
--- a/plugins/calendar/drivers/caldav/caldav_calendar.php
+++ b/plugins/calendar/drivers/caldav/caldav_calendar.php
@@ -549,8 +549,10 @@ class caldav_calendar extends kolab_storage_dav_folder
     protected function get_links($uid)
     {
         return []; // TODO
+        /*
         $storage = kolab_storage_config::get_instance();
         return $storage->get_object_links($uid);
+        */
     }
 
     /**
@@ -559,8 +561,10 @@ class caldav_calendar extends kolab_storage_dav_folder
     protected function save_links($uid, $links)
     {
         return false; // TODO
+        /*
         $storage = kolab_storage_config::get_instance();
         return $storage->save_object_links($uid, (array) $links);
+        */
     }
 
     /**
diff --git a/plugins/calendar/drivers/caldav/caldav_driver.php b/plugins/calendar/drivers/caldav/caldav_driver.php
index 85727b0b..43aa3b59 100644
--- a/plugins/calendar/drivers/caldav/caldav_driver.php
+++ b/plugins/calendar/drivers/caldav/caldav_driver.php
@@ -121,8 +121,8 @@ class caldav_driver extends kolab_driver
     /**
      * Get a list of available calendars from this source.
      *
-     * @param int    $filter Bitmask defining filter criterias
-     * @param object $tree   Reference to hierarchical folder tree object
+     * @param int                           $filter Bitmask defining filter criterias
+     * @param ?kolab_storage_folder_virtual $tree   Reference to hierarchical folder tree object
      *
      * @return array List of calendars
      */
@@ -136,9 +136,9 @@ class caldav_driver extends kolab_driver
 
         // include virtual folders for a full folder tree
         /*
-                if (!is_null($tree)) {
-                    $folders = $this->storage->folder_hierarchy($folders, $tree);
-                }
+        if (!is_null($tree)) {
+            $folders = $this->storage->folder_hierarchy($folders, $tree);
+        }
         */
         $parents = array_keys($this->calendars);
 
@@ -189,7 +189,7 @@ class caldav_driver extends kolab_driver
                 'title'     => '', // $cal->get_title(),
                 'color'     => $cal->get_color(),
                 'editable'  => $cal->editable,
-                'group'     => $is_user ? 'other user' : $cal->get_namespace(),
+                'group'     => $is_user ? 'other user' : $cal->get_namespace(), // @phpstan-ignore-line
                 'active'    => !isset($prefs[$cal->id]['active']) || !empty($prefs[$cal->id]['active']),
                 'owner'     => $cal->get_owner(),
                 'removable' => !$cal->default,
@@ -198,6 +198,7 @@ class caldav_driver extends kolab_driver
                 'driver' => 'caldav',
             ];
 
+            // @phpstan-ignore-next-line
             if (!$is_user) {
                 $calendars[$cal->id] += [
                     'default'    => $cal->default,
diff --git a/plugins/calendar/drivers/calendar_driver.php b/plugins/calendar/drivers/calendar_driver.php
index f493fc41..0b7eea80 100644
--- a/plugins/calendar/drivers/calendar_driver.php
+++ b/plugins/calendar/drivers/calendar_driver.php
@@ -430,9 +430,9 @@ abstract class calendar_driver
     /**
      * Build a struct representing the given message reference
      *
-     * @param object|string $uri_or_headers rcube_message_header instance holding the message headers
-     *                         or an URI from a stored link referencing a mail message.
-     * @param string $folder  IMAP folder the message resides in
+     * @param rcube_message_header|string $uri_or_headers An object holding the message headers
+     *                                                    or an URI from a stored link referencing a mail message.
+     * @param string                      $folder         IMAP folder the message resides in
      *
      * @return array|false An struct referencing the given IMAP message
      */
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index d7c45094..e3a72d4e 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -137,8 +137,8 @@ class kolab_driver extends calendar_driver
     /**
      * Get a list of available calendars from this source
      *
-     * @param int    $filter Bitmask defining filter criterias
-     * @param object $tree   Reference to hierarchical folder tree object
+     * @param int                           $filter Bitmask defining filter criterias
+     * @param ?kolab_storage_folder_virtual $tree   Reference to hierarchical folder tree object
      *
      * @return array List of calendars
      */
@@ -489,16 +489,15 @@ class kolab_driver extends calendar_driver
                     }
                 }
             }
+
             return $ret;
-        } else {
-            // save state in local prefs
-            $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
-            $prefs['kolab_calendars'][$prop['id']]['active'] = !empty($prop['active']);
-            $this->rc->user->save_prefs($prefs);
-            return true;
         }
 
-        return false;
+        // save state in local prefs
+        $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
+        $prefs['kolab_calendars'][$prop['id']]['active'] = !empty($prop['active']);
+        $this->rc->user->save_prefs($prefs);
+        return true;
     }
 
     /**
@@ -2035,7 +2034,8 @@ class kolab_driver extends calendar_driver
 
             unset($request, $response);
         } catch (Exception $e) {
-            PEAR::raiseError("Error fetching free/busy information: " . $e->getMessage());
+            rcube::raise_error($e);
+            return false;
         }
 
         // get free-busy url from contacts
@@ -2328,7 +2328,7 @@ class kolab_driver extends calendar_driver
             $special_changes = [];
 
             // map kolab event properties to keys the client expects
-            array_walk($result['changes'], function (&$change, $i) use ($keymap, $prop_keymaps, $special_changes) {
+            array_walk($result['changes'], function (&$change, $i) use ($keymap, $prop_keymaps, &$special_changes) {
                 if (array_key_exists($change['property'], $keymap)) {
                     $change['property'] = $keymap[$change['property']];
                 }
diff --git a/plugins/calendar/lib/calendar_ui.php b/plugins/calendar/lib/calendar_ui.php
index 68f987a3..476f5572 100644
--- a/plugins/calendar/lib/calendar_ui.php
+++ b/plugins/calendar/lib/calendar_ui.php
@@ -227,6 +227,7 @@ class calendar_ui
         $calendars = $this->cal->driver->list_calendars(0, $tree);
 
         // walk folder tree
+        // @phpstan-ignore-next-line
         if (is_object($tree)) {
             $html = $this->list_tree_html($tree, $calendars, $jsenv, $attrib);
 
@@ -284,7 +285,7 @@ class calendar_ui
             if (!empty($folder->children)) {
                 $content .= html::tag(
                     'ul',
-                    ['style' => $is_collapsed ? "display:none;" : null],
+                    ['style' => $is_collapsed ? "display:none;" : null], // @phpstan-ignore-line
                     $this->list_tree_html($folder, $data, $jsenv, $attrib)
                 );
             }
diff --git a/plugins/kolab_2fa/lib/Kolab2FA/Storage/LDAP.php b/plugins/kolab_2fa/lib/Kolab2FA/Storage/LDAP.php
index dbc09ed9..36727f11 100644
--- a/plugins/kolab_2fa/lib/Kolab2FA/Storage/LDAP.php
+++ b/plugins/kolab_2fa/lib/Kolab2FA/Storage/LDAP.php
@@ -33,7 +33,6 @@ class LDAP extends Base
     private $cache = [];
     private $ldapcache = [];
     private $conn;
-    private $error;
     private $ready = false;
 
 
@@ -53,7 +52,7 @@ class LDAP extends Base
         $this->ready = $this->conn->bind($bind_dn, $bind_pass);
 
         if (!$this->ready) {
-            throw new Exception("LDAP storage not ready: " . $this->error);
+            throw new Exception("LDAP storage not ready");
         }
     }
 
diff --git a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
index a2dbbad5..879ac859 100644
--- a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
+++ b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
@@ -87,7 +87,6 @@ class carddav_contacts extends rcube_addressbook
     private $filter;
     private $result;
     private $namespace;
-    private $action;
 
     // list of fields used for searching in "All fields" mode
     private $search_fields = [
@@ -136,8 +135,6 @@ class carddav_contacts extends rcube_addressbook
                 }
             }
         }
-
-        $this->action = rcmail::get_instance()->action;
     }
 
     /**
diff --git a/plugins/kolab_chat/drivers/mattermost.php b/plugins/kolab_chat/drivers/mattermost.php
index 8bac1f1d..f1a2644b 100644
--- a/plugins/kolab_chat/drivers/mattermost.php
+++ b/plugins/kolab_chat/drivers/mattermost.php
@@ -65,7 +65,7 @@ class kolab_chat_mattermost
                     $team = $_channel['team_name'];
                 }
 
-                if ($channel && $team) {
+                if ($team) {
                     $url .= '/' . urlencode($team) . '/channels/' . urlencode($channel);
                 }
             }
diff --git a/plugins/kolab_files/kolab_files.php b/plugins/kolab_files/kolab_files.php
index c8b32d54..a0edfa74 100644
--- a/plugins/kolab_files/kolab_files.php
+++ b/plugins/kolab_files/kolab_files.php
@@ -148,7 +148,7 @@ class kolab_files extends rcube_plugin
 
         require_once $this->home . '/lib/kolab_files_autocomplete.php';
 
-        new kolab_files_autocomplete($this);
+        new kolab_files_autocomplete();
     }
 
     /**
diff --git a/plugins/kolab_files/lib/kolab_files_autocomplete.php b/plugins/kolab_files/lib/kolab_files_autocomplete.php
index be60d514..55fbb032 100644
--- a/plugins/kolab_files/lib/kolab_files_autocomplete.php
+++ b/plugins/kolab_files/lib/kolab_files_autocomplete.php
@@ -25,17 +25,15 @@
 class kolab_files_autocomplete
 {
     private $ldap;
-    private $plugin;
     private $rc;
 
 
     /**
      * Class constructor
      */
-    public function __construct($plugin)
+    public function __construct()
     {
-        $this->plugin = $plugin;
-        $this->rc     = rcmail::get_instance();
+        $this->rc = rcmail::get_instance();
 
         $search = rcube_utils::get_input_value('_search', rcube_utils::INPUT_GPC, true);
         $reqid  = rcube_utils::get_input_value('_reqid', rcube_utils::INPUT_GPC);
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index a79e9ee8..0a8ce533 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -106,7 +106,7 @@ class kolab_folders extends rcube_plugin
         }
 
         // Create default folders
-        if ($args['root'] == '' && $args['name'] = '*') {
+        if ($args['root'] == '' && $args['name'] == '*') {
             $this->create_default_folders($folders, $args['filter'], $folderdata, $args['mode'] == 'LSUB');
         }
 
diff --git a/plugins/kolab_notes/kolab_notes.php b/plugins/kolab_notes/kolab_notes.php
index 3285bc56..9ebaa977 100644
--- a/plugins/kolab_notes/kolab_notes.php
+++ b/plugins/kolab_notes/kolab_notes.php
@@ -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 += $jsenv[$id] ?? [];
+                    $prop += $jsenv[$id] ?? []; // @phpstan-ignore-line
                     $prop['editname'] = $editname;
                     $prop['html'] = $html;
 
diff --git a/plugins/kolab_notes/kolab_notes_ui.php b/plugins/kolab_notes/kolab_notes_ui.php
index d782d38d..35ae135a 100644
--- a/plugins/kolab_notes/kolab_notes_ui.php
+++ b/plugins/kolab_notes/kolab_notes_ui.php
@@ -2,7 +2,6 @@
 
 class kolab_notes_ui
 {
-    private $action;
     private $folder;
     private $rc;
     private $plugin;
@@ -91,6 +90,7 @@ class kolab_notes_ui
         $lists = $this->plugin->get_lists($tree);
         $jsenv = [];
 
+        // @phpstan-ignore-next-line
         if (is_object($tree)) {
             $html = $this->folder_tree_html($tree, $lists, $jsenv, $attrib);
         } else {
@@ -139,7 +139,7 @@ class kolab_notes_ui
             if (!empty($folder->children)) {
                 $content .= html::tag(
                     'ul',
-                    ['style' => ($is_collapsed ? "display:none;" : null)],
+                    ['style' => ($is_collapsed ? "display:none;" : null)], // @phpstan-ignore-line
                     $this->folder_tree_html($folder, $data, $jsenv, $attrib)
                 );
             }
@@ -187,6 +187,7 @@ class kolab_notes_ui
 
         $label_id = 'nl:' . $id;
         $attr = !empty($prop['virtual']) ? ['tabindex' => '0'] : ['href' => $this->rc->url(['_list' => $id])];
+
         return html::div(
             implode(' ', $classes),
             html::a($attr + ['class' => 'listname', 'title' => $title, 'id' => $label_id], $prop['listname'] ?: $prop['name']) .
@@ -213,8 +214,6 @@ class kolab_notes_ui
                 )
             )
         );
-
-        return '';
     }
 
     public function listing($attrib)
@@ -301,7 +300,6 @@ class kolab_notes_ui
      */
     public function list_editform($action, $list, $folder)
     {
-        $this->action = $action;
         $this->list   = $list;
         $this->folder = is_object($folder) ? $folder->name : ''; // UTF7;
 
diff --git a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
index 41b2ed3a..f07a7869 100644
--- a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
+++ b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
@@ -831,8 +831,7 @@ class libcalendaring_vcalendar implements Iterator
                         $busyEnd = DateTimeParser::parse($busyEnd);
                         if ($busyEnd instanceof \DateInterval) {
                             $tmp = clone $busyStart;
-                            $tmp->add($busyEnd);
-                            $busyEnd = $tmp;
+                            $busyEnd = $tmp->add($busyEnd);
                         }
 
                         if ($busyEnd && $busyEnd > $busyStart) {
@@ -895,7 +894,7 @@ class libcalendaring_vcalendar implements Iterator
             foreach ($prop->getParts() as $val) {
                 try {
                     [$start, $end] = explode('/', $val);
-                    $start = DateTimeParser::parseDateTime($start);
+                    $start = self::toDateTime(DateTimeParser::parseDateTime($start));
 
                     // This is a duration value.
                     if ($end[0] === 'P') {
@@ -903,10 +902,10 @@ class libcalendaring_vcalendar implements Iterator
                         $end = clone $start;
                         $end->add($dur);
                     } else {
-                        $end = DateTimeParser::parseDateTime($end);
+                        $end = self::toDateTime(DateTimeParser::parseDateTime($end));
                     }
 
-                    $dt[] = [self::toDateTime($start), self::toDateTime($end)];
+                    $dt[] = [$start, $end];
                 } catch (Exception $e) {
                     // ignore single date parse errors
                 }
@@ -1080,10 +1079,10 @@ class libcalendaring_vcalendar implements Iterator
     /**
      * Build a valid iCal format block from the given event
      *
-     * @param array     $event          Hash array with event/task properties from libkolab
+     * @param array                       $event          Hash array with event/task properties from libkolab
      * @param VObject\Component\VCalendar $vcal VCalendar object to append event to or false for directly sending data to stdout
-     * @param ?callable $get_attachment Optional callback function to fetch attachment contents
-     * @param object    $recurrence_id  RECURRENCE-ID property when serializing a recurrence exception
+     * @param ?callable                   $get_attachment Optional callback function to fetch attachment contents
+     * @param VObject\Property            $recurrence_id  RECURRENCE-ID property when serializing a recurrence exception
      */
     private function _to_ical($event, $vcal, $get_attachment, $recurrence_id = null)
     {
@@ -1517,6 +1516,8 @@ class libcalendaring_vcalendar implements Iterator
 
     /**
      * Convert DateTime into libcalendaring_datetime
+     *
+     * @return libcalendaring_datetime
      */
     private static function toDateTime($date)
     {
diff --git a/plugins/libkolab/lib/kolab_attachments_handler.php b/plugins/libkolab/lib/kolab_attachments_handler.php
index b4d8be26..92453761 100644
--- a/plugins/libkolab/lib/kolab_attachments_handler.php
+++ b/plugins/libkolab/lib/kolab_attachments_handler.php
@@ -164,8 +164,7 @@ class kolab_attachments_handler
                     }
 
                     if (!empty($_SESSION[$session_key . '_deleteicon'])
-                        && ($icon = $_SESSION[$session_key . '_deleteicon'])
-                        && is_file($icon)
+                        && is_file($icon = $_SESSION[$session_key . '_deleteicon'])
                     ) {
                         $button = html::img([
                             'src' => $icon,
diff --git a/plugins/libkolab/lib/kolab_bonnie_api.php b/plugins/libkolab/lib/kolab_bonnie_api.php
index 681b40d3..c2ee6a90 100644
--- a/plugins/libkolab/lib/kolab_bonnie_api.php
+++ b/plugins/libkolab/lib/kolab_bonnie_api.php
@@ -27,7 +27,6 @@ class kolab_bonnie_api
 {
     public $ready = false;
 
-    private $config = [];
     private $client = null;
 
 
@@ -36,8 +35,6 @@ class kolab_bonnie_api
      */
     public function __construct($config)
     {
-        $this->config = $config;
-
         $this->client = new kolab_bonnie_api_client($config['uri'], $config['timeout'] ?: 30, (bool)$config['debug']);
 
         $this->client->set_secret($config['secret']);
diff --git a/plugins/libkolab/lib/kolab_date_recurrence.php b/plugins/libkolab/lib/kolab_date_recurrence.php
index d64f4250..3cd9dbe8 100644
--- a/plugins/libkolab/lib/kolab_date_recurrence.php
+++ b/plugins/libkolab/lib/kolab_date_recurrence.php
@@ -85,7 +85,8 @@ class kolab_date_recurrence
         $time = false;
 
         if ($this->engine && $this->next) {
-            if (($cnext = new cDateTime($this->engine->getNextOccurence($this->cnext))) && $cnext->isValid()) {
+            $cnext = new cDateTime($this->engine->getNextOccurence($this->cnext));
+            if ($cnext->isValid()) {
                 $next = kolab_format::php_datetime($cnext, $this->start->getTimezone());
                 $time = $timestamp ? $next->format('U') : $next;
 
diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index 2d9088be..3e453458 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -165,7 +165,7 @@ abstract class kolab_format
      * @param string $version Format version
      * @param string $xmldata Cached xml data to initialize with
      *
-     * @return kolab_format
+     * @return kolab_format|PEAR_Error
      */
     public static function factory($type, $version = '3.0', $xmldata = null)
     {
@@ -174,6 +174,7 @@ abstract class kolab_format
         }
 
         if (!self::supports($version)) {
+            // @phpstan-ignore-next-line
             return PEAR::raiseError("No support for Kolab format version " . $version);
         }
 
@@ -184,6 +185,7 @@ abstract class kolab_format
             return new $classname($xmldata, $version);
         }
 
+        // @phpstan-ignore-next-line
         return PEAR::raiseError("Failed to load Kolab Format wrapper for type " . $type);
     }
 
diff --git a/plugins/libkolab/lib/kolab_format_event.php b/plugins/libkolab/lib/kolab_format_event.php
index 1cb2f5da..a69901d1 100644
--- a/plugins/libkolab/lib/kolab_format_event.php
+++ b/plugins/libkolab/lib/kolab_format_event.php
@@ -220,7 +220,7 @@ class kolab_format_event extends kolab_format_xcal
             $object['exceptions'] = $recurrence_exceptions;
 
             // also link with recurrence.EXCEPTIONS for compatibility
-            if (!empty($object['recurrence']) && isset($object['exceptions'])) {
+            if (!empty($object['recurrence'])) {
                 $object['recurrence']['EXCEPTIONS'] = &$object['exceptions'];
             }
         }
diff --git a/plugins/libkolab/lib/kolab_ldap.php b/plugins/libkolab/lib/kolab_ldap.php
index 0ce9e408..195c9d1e 100644
--- a/plugins/libkolab/lib/kolab_ldap.php
+++ b/plugins/libkolab/lib/kolab_ldap.php
@@ -30,7 +30,6 @@ class kolab_ldap extends rcube_ldap_generic
     private $debug = false;
     private $fieldmap = [];
     private $parse_replaces = [];
-    private $rcache;
     private $ready = false;
 
 
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 8c0c0ddb..54e248a5 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -1047,8 +1047,8 @@ class kolab_storage
     /**
      * Check the folder tree and add the missing parents as virtual folders
      *
-     * @param array  $folders Folders list
-     * @param object $tree    Reference to the root node of the folder tree
+     * @param array                         $folders Folders list
+     * @param ?kolab_storage_folder_virtual $tree    Reference to the root node of the folder tree
      *
      * @return array Flat folders list
      */
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index 924429fe..cbf64db3 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -414,6 +414,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
                         $object['_formatobj']->get_attachments($object);
                     }
 
+                    // @phpstan-ignore-next-line
                     foreach ($object['_attachments'] as $attach) {
                         if ($attach['id'] == $part) {
                             if ($print) {
@@ -779,7 +780,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
                     $object['recurrence']['UNTIL'] = $untildate;
                     unset($object['recurrence']['COUNT']);
                 } else {
-                    if (!$exdates[$exception['start']->format('Y-m-d')]) {
+                    if (empty($exdates[$exception['start']->format('Y-m-d')])) {
                         $object['recurrence']['EXDATE'][] = clone $exception['start'];
                     }
                     unset($exception['recurrence']);
@@ -1023,6 +1024,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
             if (empty($att['content']) && !empty($att['id'])) {
                 // @TODO: use IMAP CATENATE to skip attachment fetch+push operation
                 $msguid = !empty($object['_copyfrom']) ? $object['_copyfrom'] : (!empty($object['_msguid']) ? $object['_msguid'] : $object['uid']);
+                // @phpstan-ignore-next-line
                 if ($is_file) {
                     $att['path'] = tempnam($temp_dir, 'rcmAttmnt');
                     if (($fp = fopen($att['path'], 'w')) && $this->get_attachment($msguid, $att['id'], $object['_mailbox'], false, $fp, true)) {
@@ -1043,6 +1045,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
             // directly to IMAP, see rcube_imap_generic::append().
             // I.e. use file handles where possible
             if (!empty($att['path'])) {
+                // @phpstan-ignore-next-line
                 if ($is_file && $binary) {
                     $files[] = fopen($att['path'], 'r');
                     $mime->addAttachment($marker, $att['mimetype'], $name, false, $encoding, 'attachment', '', '', '', null, null, '', RCUBE_CHARSET, $headers);
@@ -1050,6 +1053,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
                     $mime->addAttachment($att['path'], $att['mimetype'], $name, true, $encoding, 'attachment', '', '', '', null, null, '', RCUBE_CHARSET, $headers);
                 }
             } else {
+                // @phpstan-ignore-next-line
                 if (is_resource($att['content']) && $is_file && $binary) {
                     $files[] = $att['content'];
                     $mime->addAttachment($marker, $att['mimetype'], $name, false, $encoding, 'attachment', '', '', '', null, null, '', RCUBE_CHARSET, $headers);
@@ -1086,6 +1090,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
             $message = $tmp;
         }
         // write complete message body into temp file
+        // @phpstan-ignore-next-line
         elseif ($is_file) {
             // use common temp dir
             $body_file = tempnam($temp_dir, 'rcmMsg');
@@ -1141,6 +1146,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
         }
 
         if ($result instanceof PEAR_Error) {
+            // @phpstan-ignore-next-line
             return PEAR::raiseError(
                 sprintf("Failed triggering folder %s. Error was: %s", $this->name, $result->getMessage())
             );
@@ -1171,6 +1177,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
             $result = $request->send();
             // rcube::write_log('trigger', $result->getBody());
         } catch (Exception $e) {
+            // @phpstan-ignore-next-line
             return PEAR::raiseError($e->getMessage());
         }
 
diff --git a/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php b/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
index 273fd4ad..9437700e 100644
--- a/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
+++ b/plugins/tasklist/drivers/caldav/tasklist_caldav_driver.php
@@ -37,7 +37,6 @@ class tasklist_caldav_driver extends tasklist_driver
     private $lists;
     private $folders = [];
     private $tasks   = [];
-    private $tags    = [];
     private $bonnie_api = false;
 
 
@@ -922,29 +921,28 @@ class tasklist_caldav_driver extends tasklist_driver
      *
      * @return array (uid,folder,msguid) tuple
      */
+    /*
     private function _resolve_task_identity($prop)
     {
-        /*
-                $mailbox = $msguid = null;
+        $mailbox = $msguid = null;
 
-                $this->_parse_id($prop);
-                $uid     = $prop['uid'];
-                $list_id = $prop['list'];
+        $this->_parse_id($prop);
+        $uid     = $prop['uid'];
+        $list_id = $prop['list'];
 
-                if ($folder = $this->get_folder($list_id)) {
-                    $mailbox = $folder->get_mailbox_id();
+        if ($folder = $this->get_folder($list_id)) {
+            $mailbox = $folder->get_mailbox_id();
 
-                    // get task object from storage in order to get the real object uid an msguid
-                    if ($rec = $folder->get_object($uid)) {
-                        $msguid = $rec['_msguid'];
-                        $uid = $rec['uid'];
-                    }
-                }
+            // get task object from storage in order to get the real object uid an msguid
+            if ($rec = $folder->get_object($uid)) {
+                $msguid = $rec['_msguid'];
+                $uid = $rec['uid'];
+            }
+        }
 
-                return array($uid, $mailbox, $msguid);
-        */
-        return [];
+        return array($uid, $mailbox, $msguid);
     }
+    */
 
     /**
      * Get a list of pending alarms to be displayed to the user
diff --git a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
index 1b6814e8..2f7c59bd 100644
--- a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
+++ b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
@@ -930,7 +930,7 @@ class tasklist_kolab_driver extends tasklist_driver
             $special_changes = [];
 
             // map kolab event properties to keys the client expects
-            array_walk($result['changes'], function (&$change, $i) use ($keymap, $prop_keymaps, $special_changes) {
+            array_walk($result['changes'], function (&$change, $i) use ($keymap, $prop_keymaps, &$special_changes) {
                 if (array_key_exists($change['property'], $keymap)) {
                     $change['property'] = $keymap[$change['property']];
                 }
diff --git a/plugins/tasklist/tasklist.php b/plugins/tasklist/tasklist.php
index 6b143fb5..e9236291 100644
--- a/plugins/tasklist/tasklist.php
+++ b/plugins/tasklist/tasklist.php
@@ -652,7 +652,8 @@ class tasklist extends rcube_plugin
             }
 
             // yes, raw input matched a (valid) date
-            if (strlen($date_str) && strtotime($date_str) && ($date = new DateTime($date_str, $this->timezone))) {
+            if (strlen($date_str) && strtotime($date_str)) {
+                $date = new DateTime($date_str, $this->timezone);
                 $rec['date'] = $date->format('Y-m-d');
                 if (!empty($has_time)) {
                     $rec['time'] = $date->format('H:i');
@@ -983,7 +984,7 @@ class tasklist extends rcube_plugin
         $ignore = ['changed' => 1, 'attachments' => 1];
 
         foreach (array_unique(array_merge(array_keys($a), array_keys($b))) as $key) {
-            if (!$ignore[$key] && $a[$key] != $b[$key]) {
+            if (empty($ignore[$key]) && $a[$key] != $b[$key]) {
                 $diff[] = $key;
             }
         }
@@ -1026,7 +1027,7 @@ class tasklist extends rcube_plugin
                     if (empty($list['_reload'])) {
                         $this->load_ui();
                         $list['html'] = $this->ui->tasklist_list_item($insert_id, $list, $jsenv);
-                        $list += $jsenv[$insert_id] ?? [];
+                        $list += $jsenv[$insert_id] ?? []; // @phpstan-ignore-line
                     }
                     $this->rc->output->command('plugin.insert_tasklist', $list);
                     $success = true;
@@ -1063,7 +1064,7 @@ 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 += $jsenv[$id] ?? [];
+                    $prop += $jsenv[$id] ?? []; // @phpstan-ignore-line
                     $prop['editname'] = $editname;
                     $prop['html'] = $html;
 
@@ -2274,7 +2275,7 @@ class tasklist extends rcube_plugin
                     } else {
                         $error_msg = $this->gettext('newerversionexists');
                     }
-                } elseif (!$existing && ($status != 'declined' || $this->rc->config->get('kolab_invitation_tasklists'))) {
+                } elseif ($status != 'declined' || $this->rc->config->get('kolab_invitation_tasklists')) {
                     $success = $this->driver->create_task($task);
                 } elseif ($status == 'declined') {
                     $error_msg = null;
diff --git a/plugins/tasklist/tasklist_ui.php b/plugins/tasklist/tasklist_ui.php
index df25f981..40f9109d 100644
--- a/plugins/tasklist/tasklist_ui.php
+++ b/plugins/tasklist/tasklist_ui.php
@@ -176,6 +176,7 @@ class tasklist_ui
         }
 
         // walk folder tree
+        // @phpstan-ignore-next-line FIXME: does it make sense?
         if (is_object($tree)) {
             $html = $this->list_tree_html($tree, $lists, $jsenv, $attrib);
         } else {
@@ -224,7 +225,7 @@ class tasklist_ui
             if (!empty($folder->children)) {
                 $content .= html::tag(
                     'ul',
-                    ['style' => ($is_collapsed ? "display:none;" : null)],
+                    ['style' => ($is_collapsed ? "display:none;" : null)], // @phpstan-ignore-line
                     $this->list_tree_html($folder, $data, $jsenv, $attrib)
                 );
             }
@@ -267,7 +268,7 @@ class tasklist_ui
         if (!empty($prop['title'])) {
             $title = $prop['title'];
         } elseif (empty($prop['listname']) || $prop['name'] != $prop['listname'] || strlen($prop['name']) > 25) {
-            html_entity_decode($prop['name'], ENT_COMPAT, RCUBE_CHARSET);
+            $title = html_entity_decode($prop['name'], ENT_COMPAT, RCUBE_CHARSET);
         }
 
         if (!empty($prop['virtual'])) {