diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index f5671644..3d8b5cc6 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -812,7 +812,7 @@ class kolab_calendar extends kolab_storage_folder_api
 
             // Modify invitation status class name, when invitation calendars are disabled
             // we'll use opacity only for declined/needs-action events
-            $record['className'] = str_replace('-invitation', '', $record['className']);
+            $record['className'] = str_replace('-invitation', '', $record['className'] ?? '');
         }
 
         // add instance identifier to first occurrence (master event)
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
index c125e334..8c9216c1 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
@@ -778,14 +778,17 @@ class kolab_contacts extends rcube_addressbook
                 }
                 else {
                     // remove from distribution lists
-                    foreach ((array) $this->groupmembers[$id] as $gid) {
-                        if (!$is_mailto || $gid == $this->gid) {
-                            $this->remove_from_group($gid, $id);
+                    if (!empty($this->groupmembers[$id])) {
+                        foreach ((array) $this->groupmembers[$id] as $gid) {
+                            if (!$is_mailto || $gid == $this->gid) {
+                                $this->remove_from_group($gid, $id);
+                            }
                         }
+
+                        // clear internal cache
+                        unset($this->groupmembers[$id]);
                     }
 
-                    // clear internal cache
-                    unset($this->groupmembers[$id]);
                     $count++;
                 }
             }
@@ -1398,7 +1401,7 @@ class kolab_contacts extends rcube_addressbook
                 'birthday' => '',
                 'anniversary' => '',
                 'freebusyurl' => '',
-                'photo' => $contact['photo']
+                'photo' => $contact['photo'] ?? null
             );
     }
 }
diff --git a/plugins/kolab_delegation/kolab_delegation.php b/plugins/kolab_delegation/kolab_delegation.php
index 0b074774..518c10f5 100644
--- a/plugins/kolab_delegation/kolab_delegation.php
+++ b/plugins/kolab_delegation/kolab_delegation.php
@@ -473,7 +473,7 @@ class kolab_delegation extends rcube_plugin
             $delegate = $engine->delegate_get($id);
         }
 
-        if ($delegate) {
+        if (!empty($delegate)) {
             $input = new html_hiddenfield(array('name' => $field_id, 'id' => $field_id, 'size' => 40));
             $input = rcube::Q($delegate['name']) . $input->show($id);
 
@@ -511,7 +511,7 @@ class kolab_delegation extends rcube_plugin
             $delegate = $engine->delegate_get($id);
         }
 
-        $folder_data   = $engine->list_folders($delegate['uid']);
+        $folder_data   = $engine->list_folders(!empty($delegate) ? $delegate['uid'] : null);
         $use_fieldsets = rcube_utils::get_boolean($attrib['use-fieldsets']);
         $rights        = array();
         $folder_groups = array();
@@ -521,6 +521,8 @@ class kolab_delegation extends rcube_plugin
             $rights[$folder_name] = $folder['rights'];
         }
 
+        $html = '';
+
         // build block for every folder type
         foreach ($folder_groups as $type => $group) {
             if (empty($group)) {
@@ -550,8 +552,8 @@ class kolab_delegation extends rcube_plugin
     private function delegate_folders_block($a_folders, $attrib, $rights)
     {
         $path      = 'plugins/kolab_delegation/' . $this->skin_path . '/';
-        $read_ico  = $attrib['readicon'] ? html::img(array('src' =>  $path . $attrib['readicon'], 'title' => $this->gettext('read'))) : '';
-        $write_ico = $attrib['writeicon'] ? html::img(array('src' => $path . $attrib['writeicon'], 'title' => $this->gettext('write'))) : '';
+        $read_ico  = !empty($attrib['readicon']) ? html::img(array('src' =>  $path . $attrib['readicon'], 'title' => $this->gettext('read'))) : '';
+        $write_ico = !empty($attrib['writeicon']) ? html::img(array('src' => $path . $attrib['writeicon'], 'title' => $this->gettext('write'))) : '';
 
         $table = new html_table(array('cellspacing' => 0, 'class' => 'table-striped'));
         $table->add_header(array('class' => 'read checkbox-cell', 'title' => $this->gettext('read'), 'tabindex' => 0), $read_ico);
diff --git a/plugins/kolab_delegation/kolab_delegation_engine.php b/plugins/kolab_delegation/kolab_delegation_engine.php
index b5ad4a6c..c8884b89 100644
--- a/plugins/kolab_delegation/kolab_delegation_engine.php
+++ b/plugins/kolab_delegation/kolab_delegation_engine.php
@@ -38,6 +38,7 @@ class kolab_delegation_engine
     private $ldap_dn;
     private $cache = array();
     private $folder_types = array('mail', 'event', 'task');
+    private $supported;
 
     const ACL_READ  = 1;
     const ACL_WRITE = 2;
@@ -378,7 +379,7 @@ class kolab_delegation_engine
         // Definition of read and write ACL
         $right_types = $this->right_types();
 
-        $delegate_lc = strtolower($delegate);
+        $delegate_lc = strtolower((string) $delegate);
 
         foreach ($folders as $folder) {
             // get only folders in personal namespace
@@ -387,8 +388,8 @@ class kolab_delegation_engine
             }
 
             $rights = null;
-            $type   = $metadata[$folder] ?: 'mail';
-            list($class, $subclass) = explode('.', $type);
+            $type   = !empty($metadata[$folder]) ? $metadata[$folder] : 'mail';
+            list($class, $subclass) = strpos($type, '.') ? explode('.', $type) : [$type, ''];
 
             if (!in_array($class, $this->folder_types)) {
                 continue;
@@ -398,7 +399,7 @@ class kolab_delegation_engine
             if ($delegate) {
                 // @TODO: cache ACL
                 $imap_acl = $storage->get_acl($folder);
-                if (!empty($imap_acl) && (($acl = $imap_acl[$delegate]) || ($acl = $imap_acl[$delegate_lc]))) {
+                if (!empty($imap_acl) && (($acl = ($imap_acl[$delegate] ?? null)) || ($acl = ($imap_acl[$delegate_lc] ?? null)))) {
                     if ($this->acl_compare($acl, $right_types[self::ACL_WRITE])) {
                         $rights = self::ACL_WRITE;
                     }
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index 21082d5a..a6f7fb0f 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -532,7 +532,7 @@ class kolab_folders extends rcube_plugin
     /**
      * Checks if IMAP server supports any of METADATA, ANNOTATEMORE, ANNOTATEMORE2
      *
-     * @return boolean
+     * @return bool
      */
     function metadata_support()
     {
@@ -552,7 +552,13 @@ class kolab_folders extends rcube_plugin
      */
     function get_folder_type($folder)
     {
-        return explode('.', (string)kolab_storage::folder_type($folder));
+        $type = explode('.', (string)kolab_storage::folder_type($folder));
+
+        if (!isset($type[1])) {
+            $type[1] = null;
+        }
+
+        return $type;
     }
 
     /**
@@ -561,7 +567,7 @@ class kolab_folders extends rcube_plugin
      * @param string $folder Folder name
      * @param string $type   Content type
      *
-     * @return boolean True on success
+     * @return bool True on success
      */
     function set_folder_type($folder, $type = 'mail')
     {
diff --git a/plugins/kolab_notes/kolab_notes.php b/plugins/kolab_notes/kolab_notes.php
index 9528a39f..7f282f93 100644
--- a/plugins/kolab_notes/kolab_notes.php
+++ b/plugins/kolab_notes/kolab_notes.php
@@ -484,10 +484,9 @@ class kolab_notes extends rcube_plugin
                 // post-filter search results
                 if (strlen($search)) {
                     $matches = 0;
-                    $contents = mb_strtolower(
-                        $record['title'] .
-                        ($this->is_html($record) ? strip_tags($record['description']) : $record['description'])
-                    );
+                    $desc = $this->is_html($record) ? strip_tags($record['description']) : ($record['description'] ?? '');
+                    $contents = mb_strtolower($record['title'] . $desc);
+
                     foreach ($words as $word) {
                         if (mb_strpos($contents, $word) !== false) {
                             $matches++;
@@ -615,7 +614,7 @@ class kolab_notes extends rcube_plugin
         $action = rcube_utils::get_input_value('_do', rcube_utils::INPUT_POST);
         $note   = rcube_utils::get_input_value('_data', rcube_utils::INPUT_POST, true);
 
-        $success = $silent = false;
+        $success = $silent = $refresh = false;
         switch ($action) {
             case 'new':
             case 'edit':
@@ -1266,7 +1265,9 @@ class kolab_notes extends rcube_plugin
     private function is_html($note)
     {
         // check for opening and closing <html> or <body> tags
-        return (preg_match('/<(html|body)(\s+[a-z]|>)/', $note['description'], $m) && strpos($note['description'], '</'.$m[1].'>') > 0);
+        return !empty($note['description'])
+            && preg_match('/<(html|body)(\s+[a-z]|>)/', $note['description'], $m)
+            && strpos($note['description'], '</' . $m[1] . '>') > 0;
     }
 
     /**
diff --git a/plugins/kolab_tags/lib/kolab_tags_engine.php b/plugins/kolab_tags/lib/kolab_tags_engine.php
index a350e6ee..10f8f5db 100644
--- a/plugins/kolab_tags/lib/kolab_tags_engine.php
+++ b/plugins/kolab_tags/lib/kolab_tags_engine.php
@@ -445,7 +445,7 @@ class kolab_tags_engine
                 $class  = 'rcube_result_' . ($args['threading'] ? 'thread' : 'index');
                 $result = $args['threading'] ? '* THREAD' : '* SORT';
 
-                $args['result'] = new $class($folder, $result);
+                $args['result'] = new $class($args['folder'] ?? 'INBOX', $result);
             }
         }
 
diff --git a/plugins/libkolab/lib/kolab_format_configuration.php b/plugins/libkolab/lib/kolab_format_configuration.php
index c0979c25..bb6c4b4e 100644
--- a/plugins/libkolab/lib/kolab_format_configuration.php
+++ b/plugins/libkolab/lib/kolab_format_configuration.php
@@ -65,7 +65,7 @@ class kolab_format_configuration extends kolab_format
         case 'file_driver':
             $driver = new FileDriver($object['driver'], $object['title']);
 
-            $driver->setEnabled((bool) $object['enabled']);
+            $driver->setEnabled(!empty($object['enabled']));
 
             foreach ($this->driver_settings_fields as $field) {
                 $value = $object[$field];
@@ -78,7 +78,7 @@ class kolab_format_configuration extends kolab_format
             break;
 
         case 'relation':
-            $relation = new Relation(strval($object['name']), strval($object['category']));
+            $relation = new Relation(strval($object['name'] ?? ''), strval($object['category'] ?? ''));
 
             if (!empty($object['color'])) {
                 $relation->setColor($object['color']);
diff --git a/plugins/libkolab/lib/kolab_format_distributionlist.php b/plugins/libkolab/lib/kolab_format_distributionlist.php
index 88c6f7b9..db6343e0 100644
--- a/plugins/libkolab/lib/kolab_format_distributionlist.php
+++ b/plugins/libkolab/lib/kolab_format_distributionlist.php
@@ -60,8 +60,9 @@ class kolab_format_distributionlist extends kolab_format
                 continue;
             }
 
-            if (!$seen[$key]++) {
+            if (empty($seen[$key])) {
                 $members->push($m);
+                $seen[$key] = true;
             }
             else {
                 // remove dupes for caching
diff --git a/plugins/libkolab/lib/kolab_format_event.php b/plugins/libkolab/lib/kolab_format_event.php
index 9293487a..f801bd93 100644
--- a/plugins/libkolab/lib/kolab_format_event.php
+++ b/plugins/libkolab/lib/kolab_format_event.php
@@ -239,13 +239,13 @@ class kolab_format_event extends kolab_format_xcal
         $recurrence_id_format = libkolab::recurrence_id_format($object);
         $instance_id = $recurrence_id instanceof DateTimeInterface ? $recurrence_id->format($recurrence_id_format) : strval($recurrence_id);
 
-        if ($object['recurrence_date'] instanceof DateTimeInterface) {
+        if (!empty($object['recurrence_date']) && $object['recurrence_date'] instanceof DateTimeInterface) {
             if ($object['recurrence_date']->format($recurrence_id_format) == $instance_id) {
                 $result = $object;
             }
         }
 
-        if (!$result && is_array($object['exceptions'])) {
+        if (!$result && !empty($object['exceptions']) && is_array($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 if ($exception['_instance'] == $instance_id) {
                     $result = $exception;
diff --git a/plugins/libkolab/lib/kolab_format_xcal.php b/plugins/libkolab/lib/kolab_format_xcal.php
index 88e3d9f7..e4417199 100644
--- a/plugins/libkolab/lib/kolab_format_xcal.php
+++ b/plugins/libkolab/lib/kolab_format_xcal.php
@@ -423,14 +423,14 @@ abstract class kolab_format_xcal extends kolab_format
 
         if (!empty($object['recurrence']['FREQ'])) {
             $freq     = $object['recurrence']['FREQ'];
-            $bysetpos = explode(',', $object['recurrence']['BYSETPOS']);
+            $bysetpos = isset($object['recurrence']['BYSETPOS']) ? explode(',', $object['recurrence']['BYSETPOS']) : [];
 
             $rr->setFrequency($this->rrule_type_map[$freq]);
 
             if ($object['recurrence']['INTERVAL'])
                 $rr->setInterval(intval($object['recurrence']['INTERVAL']));
 
-            if ($object['recurrence']['BYDAY']) {
+            if (!empty($object['recurrence']['BYDAY'])) {
                 $byday = new vectordaypos;
                 foreach (explode(',', $object['recurrence']['BYDAY']) as $day) {
                     $occurrence = 0;
@@ -455,31 +455,37 @@ abstract class kolab_format_xcal extends kolab_format
                 $rr->setByday($byday);
             }
 
-            if ($object['recurrence']['BYMONTHDAY']) {
+            if (!empty($object['recurrence']['BYMONTHDAY'])) {
                 $bymday = new vectori;
-                foreach (explode(',', $object['recurrence']['BYMONTHDAY']) as $day)
+                foreach (explode(',', $object['recurrence']['BYMONTHDAY']) as $day) {
                     $bymday->push(intval($day));
+                }
                 $rr->setBymonthday($bymday);
             }
 
-            if ($object['recurrence']['BYMONTH']) {
+            if (!empty($object['recurrence']['BYMONTH'])) {
                 $bymonth = new vectori;
                 foreach (explode(',', $object['recurrence']['BYMONTH']) as $month)
                     $bymonth->push(intval($month));
                 $rr->setBymonth($bymonth);
             }
 
-            if ($object['recurrence']['COUNT'])
+            if (!empty($object['recurrence']['COUNT'])) {
                 $rr->setCount(intval($object['recurrence']['COUNT']));
-            else if ($object['recurrence']['UNTIL'])
+            }
+            else if (!empty($object['recurrence']['UNTIL'])) {
                 $rr->setEnd(self::get_datetime($object['recurrence']['UNTIL'], null, true, $start_tz));
+            }
 
             if ($rr->isValid()) {
                 // add exception dates (only if recurrence rule is valid)
-                $exdates = new vectordatetime;
-                foreach ((array)$object['recurrence']['EXDATE'] as $exdate)
-                    $exdates->push(self::get_datetime($exdate, null, true, $start_tz));
-                $this->obj->setExceptionDates($exdates);
+                if (!empty($object['recurrence']['EXDATE'])) {
+                    $exdates = new vectordatetime;
+                    foreach ((array)$object['recurrence']['EXDATE'] as $exdate) {
+                        $exdates->push(self::get_datetime($exdate, null, true, $start_tz));
+                    }
+                    $this->obj->setExceptionDates($exdates);
+                }
             }
             else {
                 rcube::raise_error(array(
@@ -495,8 +501,9 @@ abstract class kolab_format_xcal extends kolab_format
         // save recurrence dates (aka RDATE)
         if (!empty($object['recurrence']['RDATE'])) {
             $rdates = new vectordatetime;
-            foreach ((array)$object['recurrence']['RDATE'] as $rdate)
+            foreach ((array)$object['recurrence']['RDATE'] as $rdate) {
                 $rdates->push(self::get_datetime($rdate, null, true, $start_tz));
+            }
             $this->obj->setRecurrenceDates($rdates);
         }
 
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index daedcb68..03cd404e 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -1273,7 +1273,7 @@ class kolab_storage
             if (self::folder_is_subscribed($folder)) {
                 return true;
             }
-            else if (!is_array($_SESSION['kolab_subscribed_folders']) || !in_array($folder, $_SESSION['kolab_subscribed_folders'])) {
+            else if (empty($_SESSION['kolab_subscribed_folders']) || !in_array($folder, $_SESSION['kolab_subscribed_folders'])) {
                 $_SESSION['kolab_subscribed_folders'][] = $folder;
                 return true;
             }
@@ -1300,7 +1300,7 @@ class kolab_storage
 
         // temporary/session subscription
         if ($temp) {
-            if (is_array($_SESSION['kolab_subscribed_folders']) && ($i = array_search($folder, $_SESSION['kolab_subscribed_folders'])) !== false) {
+            if (!empty($_SESSION['kolab_subscribed_folders']) && ($i = array_search($folder, $_SESSION['kolab_subscribed_folders'])) !== false) {
                 unset($_SESSION['kolab_subscribed_folders'][$i]);
             }
             return true;
@@ -1608,7 +1608,7 @@ class kolab_storage
             $other_ns = rtrim(self::namespace_root('other'), $delimiter);
             $path_len = count(explode($delimiter, $other_ns));
 
-            foreach ((array)self::list_folders($other_ns . $delimiter, '*', '', $subscribed) as $foldername) {
+            foreach ((array) self::list_folders($other_ns . $delimiter, '*', '', $subscribed) as $foldername) {
                 if ($foldername == 'INBOX')  // skip INBOX which is added by default
                     continue;
 
@@ -1622,16 +1622,16 @@ class kolab_storage
                 // truncate folder path to top-level folders of the 'other' namespace
                 $foldername = join($delimiter, array_slice($path, 0, $path_len + 1));
 
-                if (!$folders[$foldername]) {
+                if (empty($folders[$foldername])) {
                     $folders[$foldername] = new kolab_storage_folder_user($foldername, $other_ns);
                 }
             }
 
             // for every (subscribed) user folder, list all (unsubscribed) subfolders
             foreach ($folders as $userfolder) {
-                foreach ((array)self::list_folders($userfolder->name . $delimiter, '*', $type, false, $folderdata) as $foldername) {
-                    if (!$folders[$foldername]) {
-                        $folders[$foldername] = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
+                foreach ((array) self::list_folders($userfolder->name . $delimiter, '*', $type, false, $folderdata) as $foldername) {
+                    if (empty($folders[$foldername])) {
+                        $folders[$foldername] = new kolab_storage_folder($foldername, $type, $folderdata[$foldername] ?? null);
                         $userfolder->children[] = $folders[$foldername];
                     }
                 }
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index a61922f4..766975f8 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -609,7 +609,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
 
         // copy attachments from old message
         $copyfrom = $object['_copyfrom'] ?? ($object['_msguid'] ?? null);
-        if (!empty($copyfrom) && ($old = $this->cache->get($copyfrom, $type, $object['_mailbox']))) {
+        if (!empty($copyfrom) && ($old = $this->cache->get($copyfrom, $type, $object['_mailbox'])) && !empty($old['_attachments'])) {
             foreach ((array)$old['_attachments'] as $key => $att) {
                 if (!isset($object['_attachments'][$key])) {
                     $object['_attachments'][$key] = $old['_attachments'][$key];
@@ -1003,7 +1003,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
         foreach ((array)($object['_attachments'] ?? []) as $key => $att) {
             if (empty($att['content']) && !empty($att['id'])) {
                 // @TODO: use IMAP CATENATE to skip attachment fetch+push operation
-                $msguid = $object['_copyfrom'] ?: ($object['_msguid'] ?: $object['uid']);
+                $msguid = !empty($object['_copyfrom']) ? $object['_copyfrom'] : (!empty($object['_msguid']) ? $object['_msguid'] : $object['uid']);
                 if ($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)) {
diff --git a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
index 548cf1fe..94c025dc 100644
--- a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
+++ b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
@@ -335,15 +335,15 @@ class tasklist_kolab_driver extends tasklist_driver
         $this->_read_lists();
 
         // create list and folder instance if necesary
-        if (!$this->lists[$id]) {
+        if (empty($this->lists[$id])) {
             $folder = kolab_storage::get_folder(kolab_storage::id_decode($id));
-            if ($folder->type) {
+            if ($folder && $folder->type) {
                 $this->folders[$id] = $folder;
                 $this->lists[$id] = $this->folder_props($folder, $this->rc->config->get('kolab_tasklists', array()));
             }
         }
 
-        return $this->folders[$id];
+        return $this->folders[$id] ?? null;
     }
 
 
@@ -455,8 +455,8 @@ class tasklist_kolab_driver extends tasklist_driver
                 $ret |= $folder->activate(intval($prop['active']));
 
             // apply to child folders, too
-            if ($prop['recursive']) {
-                foreach ((array)kolab_storage::list_folders($folder->name, '*', 'task') as $subfolder) {
+            if (!empty($prop['recursive'])) {
+                foreach ((array) kolab_storage::list_folders($folder->name, '*', 'task') as $subfolder) {
                     if (isset($prop['permanent']))
                         ($prop['permanent'] ? kolab_storage::folder_subscribe($subfolder) : kolab_storage::folder_unsubscribe($subfolder));
                     if (isset($prop['active']))
@@ -640,13 +640,13 @@ class tasklist_kolab_driver extends tasklist_driver
 
         // query Kolab storage
         $query = array();
-        if ($filter['mask'] & tasklist::FILTER_MASK_COMPLETE)
+        if (!empty($filter['mask']) && $filter['mask'] & tasklist::FILTER_MASK_COMPLETE)
             $query[] = array('tags','~','x-complete');
         else if (empty($filter['since']))
             $query[] = array('tags','!~','x-complete');
 
         // full text search (only works with cache enabled)
-        if ($filter['search']) {
+        if (!empty($filter['search'])) {
             $search = mb_strtolower($filter['search']);
             foreach (rcube_utils::normalize_string($search, true) as $word) {
                 $query[] = array('words', '~', $word);