diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index e09e49a8..a6f304a0 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -945,7 +945,7 @@ class kolab_calendar extends kolab_storage_folder_api
         $out = '';
         if (is_array($prop)) {
             foreach ($prop as $key => $val) {
-                if (is_numeric($key)) {
+                if (is_numeric($key) || is_array($val)) {
                     $out .= self::_complex2string($val);
                 }
                 else if (!in_array($key, $ignorekeys)) {
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 67f83a21..76d1974d 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -227,7 +227,7 @@ class kolab_driver extends calendar_driver
                 }
             }
 
-            if ($cal->subscriptions) {
+            if (!empty($cal->subscriptions)) {
                 $calendars[$cal->id]['subscribed'] = $cal->is_subscribed();
             }
         }
diff --git a/plugins/kolab_activesync/kolab_activesync_ui.php b/plugins/kolab_activesync/kolab_activesync_ui.php
index c0a557a1..50d20033 100644
--- a/plugins/kolab_activesync/kolab_activesync_ui.php
+++ b/plugins/kolab_activesync/kolab_activesync_ui.php
@@ -109,14 +109,14 @@ class kolab_activesync_ui
 
     public function folder_subscriptions($attrib = [])
     {
-        if (!$attrib['id']) {
+        if (empty($attrib['id'])) {
             $attrib['id'] = 'foldersubscriptions';
         }
 
         // group folders by type (show only known types)
         $folder_groups = ['mail' => [], 'contact' => [], 'event' => [], 'task' => [], 'note' => []];
         $folder_types  = kolab_storage::folders_typedata();
-        $use_fieldsets = rcube_utils::get_boolean($attrib['use-fieldsets']);
+        $use_fieldsets = rcube_utils::get_boolean($attrib['use-fieldsets'] ?? '');
         $imei          = $this->device['_id'];
         $subscribed    = [];
 
diff --git a/plugins/libcalendaring/lib/libcalendaring_itip.php b/plugins/libcalendaring/lib/libcalendaring_itip.php
index 99290167..f30a235f 100644
--- a/plugins/libcalendaring/lib/libcalendaring_itip.php
+++ b/plugins/libcalendaring/lib/libcalendaring_itip.php
@@ -617,6 +617,7 @@ class libcalendaring_itip
 
         $rsvp_status  = 'unknown';
         $rsvp_buttons = '';
+        $title = '';
 
         // pass some metadata about the event and trigger the asynchronous status check
         $changed = !empty($event['changed']) && is_object($event['changed']) ? $event['changed'] : $message_date;
diff --git a/plugins/libkolab/lib/kolab_date_recurrence.php b/plugins/libkolab/lib/kolab_date_recurrence.php
index 2aaf5888..fe30f8f1 100644
--- a/plugins/libkolab/lib/kolab_date_recurrence.php
+++ b/plugins/libkolab/lib/kolab_date_recurrence.php
@@ -199,7 +199,7 @@ class kolab_date_recurrence
 
         $event['start'] = $start;
         $event['recurrence']['INTERVAL'] = $interval;
-        if ($event['recurrence']['COUNT']) {
+        if (!empty($event['recurrence']['COUNT'])) {
             // Increase count so we do not stop the loop to early
             $event['recurrence']['COUNT'] += 100;
         }
diff --git a/plugins/libkolab/lib/kolab_format_contact.php b/plugins/libkolab/lib/kolab_format_contact.php
index 82e5bee3..25d5ebf6 100644
--- a/plugins/libkolab/lib/kolab_format_contact.php
+++ b/plugins/libkolab/lib/kolab_format_contact.php
@@ -350,7 +350,7 @@ class kolab_format_contact extends kolab_format
         $teltypes = array_flip($this->phonetypes);
         for ($i=0; $i < $tels->size(); $i++) {
             $tel = $tels->get($i);
-            $object['phone'][] = array('number' => $tel->number(), 'type' => $teltypes[$tel->types()]);
+            $object['phone'][] = ['number' => $tel->number(), 'type' => $teltypes[$tel->types()] ?? null];
         }
 
         $object['notes'] = $this->obj->note();
diff --git a/plugins/libkolab/lib/kolab_format_task.php b/plugins/libkolab/lib/kolab_format_task.php
index 398c8da4..74693dd6 100644
--- a/plugins/libkolab/lib/kolab_format_task.php
+++ b/plugins/libkolab/lib/kolab_format_task.php
@@ -53,7 +53,9 @@ class kolab_format_task extends kolab_format_xcal
         // set common xcal properties
         parent::set($object);
 
-        $this->obj->setPercentComplete(intval($object['complete']));
+        $object['complete'] = (int) ($object['complete'] ?? 0);
+
+        $this->obj->setPercentComplete($object['complete']);
 
         $status = kolabformat::StatusUndefined;
         if ($object['complete'] == 100 && !array_key_exists('status', $object))
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 65d965e8..6388a92c 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -516,8 +516,8 @@ class kolab_storage
         self::setup();
 
         $folder    = rcube_charset::convert($prop['name'], RCUBE_CHARSET, 'UTF7-IMAP');
-        $oldfolder = $prop['oldname']; // UTF7
-        $parent    = $prop['parent']; // UTF7
+        $oldfolder = $prop['oldname'] ?? ''; // UTF7
+        $parent    = $prop['parent'] ?? ''; // UTF7
         $delimiter = self::$imap->get_hierarchy_delimiter();
 
         if (strlen($oldfolder)) {
@@ -1083,7 +1083,7 @@ class kolab_storage
                     array_pop($path);
                     $parent_parent = join($delim, $path);
 
-                    if (!$refs[$parent]) {
+                    if (empty($refs[$parent])) {
                         if ($folder->type && self::folder_type($parent) == $folder->type) {
                             $refs[$parent] = new kolab_storage_folder($parent, $folder->type, $folder->type);
                             $refs[$parent]->parent = $parent_parent;
@@ -1095,6 +1095,7 @@ class kolab_storage
                             $name = kolab_storage::object_name($parent);
                             $refs[$parent] = new kolab_storage_folder_virtual($parent, $name, $folder->get_namespace(), $parent_parent);
                         }
+
                         $parents[] = $refs[$parent];
                     }
                 }
@@ -1259,7 +1260,7 @@ class kolab_storage
         }
 
         return in_array($folder, self::$subscriptions) ||
-            ($temp && in_array($folder, (array)$_SESSION['kolab_subscribed_folders']));
+            ($temp && in_array($folder, $_SESSION['kolab_subscribed_folders'] ?? []));
     }
 
     /**
@@ -1727,7 +1728,7 @@ class kolab_storage
         }
 
         if (empty($user) && ($ldap = self::ldap())) {
-            $user = $ldap->get_user_record($token, $_SESSION['imap_host']);
+            $user = $ldap->get_user_record($token, $_SESSION['imap_host'] ?? '');
 
             if (!empty($user)) {
                 $keys = array('displayname', 'name', 'mail'); // supported keys
diff --git a/plugins/libkolab/lib/kolab_storage_cache_contact.php b/plugins/libkolab/lib/kolab_storage_cache_contact.php
index 754bdd3e..4bd3ec1a 100644
--- a/plugins/libkolab/lib/kolab_storage_cache_contact.php
+++ b/plugins/libkolab/lib/kolab_storage_cache_contact.php
@@ -37,7 +37,7 @@ class kolab_storage_cache_contact extends kolab_storage_cache
         $sql_data = parent::_serialize($object);
         $sql_data['type'] = $object['_type'];
 
-        $email = is_array($object['email'] ?? null) ? $object['email'][0] : ($object['email'] ?? '');
+        $email = is_array($object['email'] ?? null) ? ($object['email'][0] ?? '') : ($object['email'] ?? '');
 
         // columns for sorting
         $sql_data['name']      = rcube_charset::clean(($object['name'] ?? '') . ($object['prefix'] ?? ''));
diff --git a/plugins/libkolab/lib/kolab_storage_config.php b/plugins/libkolab/lib/kolab_storage_config.php
index e5bdf451..4deeacc8 100644
--- a/plugins/libkolab/lib/kolab_storage_config.php
+++ b/plugins/libkolab/lib/kolab_storage_config.php
@@ -552,7 +552,7 @@ class kolab_storage_config
                     $tag['members'] = array_merge($tag['members'], $members);
 
                     // add UIDs into the result
-                    $result[$folder] = array_unique(array_merge((array)$result[$folder], $uids));
+                    $result[$folder] = array_unique(array_merge((array) ($result[$folder] ?? []), $uids));
                 }
             }