diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index ac5d3086..64b3afb1 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -401,8 +401,8 @@ class kolab_calendar extends kolab_storage_folder_api
             if (count($partstat_exclude) && !empty($event['attendees'])) {
                 foreach ($event['attendees'] as $attendee) {
                     if (
-                        in_array($attendee['email'], $user_emails)
-                        && in_array($attendee['status'] ?? null, $partstat_exclude)
+                        !empty($attendee['email']) && in_array($attendee['email'], $user_emails)
+                        && !empty($attendee['status']) && in_array($attendee['status'], $partstat_exclude)
                     ) {
                         return false;
                     }
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 10d301d1..e8aaf1c0 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -180,7 +180,7 @@ class kolab_driver extends calendar_driver
 
             $parents[] = $cal->id;
 
-            if (property_exists($cal, "virtual") && $cal->virtual) {
+            if (!empty($cal->virtual)) {
                 $calendars[$cal->id] = [
                     'id'       => $cal->id,
                     'name'     => $cal->get_name(),
diff --git a/plugins/kolab_activesync/kolab_activesync_ui.php b/plugins/kolab_activesync/kolab_activesync_ui.php
index 45122b04..2ef1448a 100644
--- a/plugins/kolab_activesync/kolab_activesync_ui.php
+++ b/plugins/kolab_activesync/kolab_activesync_ui.php
@@ -27,8 +27,8 @@ class kolab_activesync_ui
 {
     private $rc;
     private $plugin;
-    private $force_subscriptions = array();
-    public  $device = array();
+    private $force_subscriptions = [];
+    public  $device = [];
 
     const SETUP_URL = 'https://kb.kolabenterprise.com/documentation/setting-up-an-activesync-client';
 
@@ -41,21 +41,21 @@ class kolab_activesync_ui
         $this->skin_path = 'plugins/kolab_activesync/' . $skin_path;
 
         $this->plugin->load_config();
-        $this->force_subscriptions = $this->rc->config->get('activesync_force_subscriptions', array());
+        $this->force_subscriptions = $this->rc->config->get('activesync_force_subscriptions', []);
 
         $this->plugin->include_stylesheet($skin_path . 'config.css');
     }
 
-    public function device_list($attrib = array())
+    public function device_list($attrib = [])
     {
-        $attrib += array('id' => 'devices-list');
+        $attrib += ['id' => 'devices-list'];
 
         $devices = $this->plugin->list_devices();
         $table   = new html_table();
 
         foreach ($devices as $id => $device) {
             $name = $device['ALIAS'] ? $device['ALIAS'] : $id;
-            $table->add_row(array('id' => 'rcmrow' . $id));
+            $table->add_row(['id' => 'rcmrow' . $id]);
             $table->add(null, html::span('devicealias', rcube::Q($name))
                 . ' ' . html::span('devicetype secondary', rcube::Q($device['TYPE'])));
         }
@@ -69,12 +69,12 @@ class kolab_activesync_ui
     }
 
 
-    public function device_config_form($attrib = array())
+    public function device_config_form($attrib = [])
     {
-        $table = new html_table(array('cols' => 2));
+        $table = new html_table(['cols' => 2]);
 
         $field_id = 'config-device-alias';
-        $input = new html_inputfield(array('name' => 'devicealias', 'id' => $field_id, 'size' => 40));
+        $input = new html_inputfield(['name' => 'devicealias', 'id' => $field_id, 'size' => 40]);
         $table->add('title', html::label($field_id, $this->plugin->gettext('devicealias')));
         $table->add(null, $input->show($this->device['ALIAS'] ? $this->device['ALIAS'] : $this->device['_id']));
 
@@ -107,43 +107,41 @@ class kolab_activesync_ui
         return false;
     }
 
-    public function folder_subscriptions($attrib = array())
+    public function folder_subscriptions($attrib = [])
     {
         if (!$attrib['id']) {
             $attrib['id'] = 'foldersubscriptions';
         }
 
         // group folders by type (show only known types)
-        $folder_groups = array('mail' => array(), 'contact' => array(), 'event' => array(), 'task' => array(), 'note' => array());
+        $folder_groups = ['mail' => [], 'contact' => [], 'event' => [], 'task' => [], 'note' => []];
         $folder_types  = kolab_storage::folders_typedata();
         $use_fieldsets = rcube_utils::get_boolean($attrib['use-fieldsets']);
         $imei          = $this->device['_id'];
-        $subscribed    = array();
+        $subscribed    = [];
 
         if ($imei) {
             $folder_meta = $this->plugin->folder_meta();
         }
 
         $devicetype = strtolower($this->device['TYPE']);
-        $device_force_subscriptions = $this->force_subscriptions[$devicetype] ?? null;
+        $device_force_subscriptions = $this->force_subscriptions[$devicetype] ?? [];
 
         foreach ($this->plugin->list_folders() as $folder) {
-            if ($folder_types[$folder] ?? null) {
+            if (!empty($folder_types[$folder])) {
                 list($type, ) = explode('.', $folder_types[$folder]);
             }
             else {
                 $type = 'mail';
             }
 
-            if (is_array($folder_groups[$type] ?? null)) {
+            if (!empty($folder_groups[$type])) {
                 $folder_groups[$type][] = $folder;
 
                 if ($device_force_subscriptions && array_key_exists($folder, $device_force_subscriptions)) {
-                    $subscribed[$folder] = intval($device_force_subscriptions[$folder] ?? null);
-                } else if (!empty($folder_meta) && ($meta = ($folder_meta[$folder] ?? null))
-                    && ($meta['FOLDER'] ?? false) && $meta['FOLDER'][$imei]['S']
-                ) {
-                    $subscribed[$folder] = intval($meta['FOLDER'][$imei]['S']);
+                    $subscribed[$folder] = intval($device_force_subscriptions[$folder]);
+                } else if (!empty($folder_meta[$folder]['FOLDER'][$imei]['S'])) {
+                    $subscribed[$folder] = intval($folder_meta[$folder]['FOLDER'][$imei]['S']);
                 }
             }
         }
@@ -176,33 +174,31 @@ class kolab_activesync_ui
     {
         $alarms = ($attrib['type'] == 'event' || $attrib['type'] == 'task');
 
-        $table = new html_table(array('cellspacing' => 0, 'class' => 'table-striped'));
-        $table->add_header(array(
+        $table = new html_table(['cellspacing' => 0, 'class' => 'table-striped']);
+        $table->add_header([
                 'class'    => 'subscription checkbox-cell',
                 'title'    => $this->plugin->gettext('synchronize'),
                 'tabindex' => 0
-            ),
-            ($attrib['syncicon'] ?? false) ? html::img(array('src' => $this->skin_path . $attrib['syncicon'])) :
-                $this->plugin->gettext('synchronize')
+            ],
+            !empty($attrib['syncicon']) ? html::img(['src' => $this->skin_path . $attrib['syncicon']]) : $this->plugin->gettext('synchronize')
         );
 
         if ($alarms) {
-            $table->add_header(array(
+            $table->add_header([
                     'class'    => 'alarm checkbox-cell',
                     'title'    => $this->plugin->gettext('withalarms'),
                     'tabindex' => 0
-                ),
-                ($attrib['alarmicon'] ?? null) ? html::img(array('src' => $this->skin_path . $attrib['alarmicon'])) :
-                    $this->plugin->gettext('withalarms')
+                ],
+                !empty($attrib['alarmicon']) ? html::img(['src' => $this->skin_path . $attrib['alarmicon']]) : $this->plugin->gettext('withalarms')
             );
         }
 
         $table->add_header('foldername', $this->plugin->gettext('folder'));
 
-        $checkbox_sync  = new html_checkbox(array('name' => 'subscribed[]', 'class' => 'subscription'));
-        $checkbox_alarm = new html_checkbox(array('name' => 'alarm[]', 'class' => 'alarm'));
+        $checkbox_sync  = new html_checkbox(['name' => 'subscribed[]', 'class' => 'subscription']);
+        $checkbox_alarm = new html_checkbox(['name' => 'alarm[]', 'class' => 'alarm']);
 
-        $names = array();
+        $names = [];
         foreach ($a_folders as $folder) {
             $foldername = $origname = kolab_storage::object_prettyname($folder);
 
@@ -219,7 +215,7 @@ class kolab_activesync_ui
 
             $folder_id = 'rcmf' . rcube_utils::html_identifier($folder);
             $names[] = $origname;
-            $classes = array('mailbox');
+            $classes = ['mailbox'];
 
             if ($folder_class = $this->rc->folder_classname($folder)) {
                 if ($this->rc->text_exists($folder_class)) {
@@ -234,12 +230,12 @@ class kolab_activesync_ui
 
             $table->add('subscription checkbox-cell', $checkbox_sync->show(
                 !empty($subscribed[$folder]) ? $folder : null,
-                array('value' => $folder, 'id' => $folder_id, 'disabled' => $disabled)));
+                ['value' => $folder, 'id' => $folder_id, 'disabled' => $disabled]));
 
             if ($alarms) {
                 $table->add('alarm checkbox-cell', $checkbox_alarm->show(
                     intval($subscribed[$folder] ?? 0) > 1 ? $folder : null,
-                    array('value' => $folder, 'id' => $folder_id.'_alarm', 'disabled' => $disabled)));
+                    ['value' => $folder, 'id' => $folder_id.'_alarm', 'disabled' => $disabled]));
             }
 
             $table->add(join(' ', $classes), html::label($folder_id, $foldername));
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
index 53147f5c..1b02733c 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts.php
@@ -166,7 +166,7 @@ class kolab_contacts extends rcube_addressbook
 
         // set localized labels for proprietary cols
         foreach ($this->coltypes as $col => $prop) {
-            if (is_string($prop['label'] ?? null)) {
+            if (!empty($prop['label']) && is_string($prop['label'])) {
                 $this->coltypes[$col]['label'] = $rcube->gettext($prop['label']);
             }
         }
@@ -1320,10 +1320,10 @@ class kolab_contacts extends rcube_addressbook
      */
     private function _from_rcube_contact($contact, $old = array())
     {
-        if (!($contact['uid'] ?? null) && ($contact['ID'] ?? null)) {
+        if (empty($contact['uid']) && !empty($contact['ID'])) {
             $contact['uid'] = $this->id2uid($contact['ID']);
         }
-        else if (!($contact['uid'] ?? null) && ($old['uid'] ?? null)) {
+        else if (empty($contact['uid']) && !empty($old['uid'])) {
             $contact['uid'] = $old['uid'];
         }
 
@@ -1380,7 +1380,7 @@ class kolab_contacts extends rcube_addressbook
         // convert one-item-array elements into string element
         // this is needed e.g. to properly import birthday field
         foreach ($this->coltypes as $type => $col_def) {
-            if (($col_def['limit'] ?? null) == 1 && is_array($contact[$type] ?? null)) {
+            if (($col_def['limit'] ?? 0) == 1 && !empty($contact[$type]) && is_array($contact[$type])) {
                 $contact[$type] = array_shift(array_filter($contact[$type]));
             }
         }
diff --git a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
index 2f4635fc..0ecedf81 100644
--- a/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
+++ b/plugins/kolab_addressbook/drivers/kolab/kolab_contacts_driver.php
@@ -242,7 +242,7 @@ class kolab_contacts_driver
      */
     public function abook_prop($id, $abook)
     {
-        if (property_exists($abook, 'virtual') && $abook->virtual) {
+        if (!empty($abook->virtual)) {
             return [
                 'id'       => $id,
                 'name'     => $abook->get_name(),
diff --git a/plugins/kolab_addressbook/kolab_addressbook.php b/plugins/kolab_addressbook/kolab_addressbook.php
index 2c239979..6e34b973 100644
--- a/plugins/kolab_addressbook/kolab_addressbook.php
+++ b/plugins/kolab_addressbook/kolab_addressbook.php
@@ -471,7 +471,7 @@ class kolab_addressbook extends rcube_plugin
     public function contact_form($p)
     {
         // none of our business
-        if (!is_object($GLOBALS['CONTACTS'] ?? null) || !is_a($GLOBALS['CONTACTS'], 'kolab_contacts')) {
+        if (empty($GLOBALS['CONTACTS']) || !($GLOBALS['CONTACTS'] instanceof kolab_contacts)) {
             return $p;
         }
 
diff --git a/plugins/kolab_files/lib/kolab_files_engine.php b/plugins/kolab_files/lib/kolab_files_engine.php
index 7c8e9c0d..311a8695 100644
--- a/plugins/kolab_files/lib/kolab_files_engine.php
+++ b/plugins/kolab_files/lib/kolab_files_engine.php
@@ -923,7 +923,7 @@ class kolab_files_engine
      */
     public function quota_display($attrib)
     {
-        if (!($attrib['id'] ?? false)) {
+        if (empty($attrib['id'])) {
             $attrib['id'] = 'rcmquotadisplay';
         }
 
@@ -1078,7 +1078,7 @@ class kolab_files_engine
     {
         $url = $this->url_srv . '/api/';
 
-        if (!property_exists($this, "request") || !$this->request) {
+        if (empty($this->request)) {
             $config = array(
                 'store_body'       => true,
                 'follow_redirects' => true,
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index b9b9f914..e68c436a 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -159,7 +159,9 @@ class kolab_folders extends rcube_plugin
 
         // Add type-based style for table rows
         // See kolab_folders::folder_class_name()
-        if ($table = ($args['table'] ?? null)) {
+        if (!empty($args['table'])) {
+            $table = $args['table'];
+
             for ($i=1, $cnt=$table->size(); $i<=$cnt; $i++) {
                 $attrib = $table->get_row_attribs($i);
                 $folder = $attrib['foldername']; // UTF7-IMAP
@@ -176,8 +178,8 @@ class kolab_folders extends rcube_plugin
         }
 
         // Add type-based class for list items
-        if (is_array($args['list'] ?? null)) {
-            foreach ((array)$args['list'] as $k => $item) {
+        if (!empty($args['list']) && is_array($args['list'])) {
+            foreach ($args['list'] as $k => $item) {
                 $folder = $item['folder_imap']; // UTF7-IMAP
                 $type   = $folderdata[$folder] ?? null;
 
@@ -597,14 +599,13 @@ class kolab_folders extends rcube_plugin
      */
     static function folder_class_name($type)
     {
-        list($ctype, $subtype) = array_pad(explode('.', $type), 2, null);
+        if ($type && strpos($type, '.')) {
+            list($ctype, $subtype) = explode('.', $type);
 
-        $class[] = 'type-' . ($ctype ? $ctype : 'mail');
+            return 'type-' . $ctype . ' subtype-' . $subtype;
+        }
 
-        if ($subtype)
-            $class[] = 'subtype-' . $subtype;
-
-        return implode(' ', $class);
+        return 'type-' . ($type ? $type : 'mail');
     }
 
     /**
@@ -745,7 +746,7 @@ class kolab_folders extends rcube_plugin
         $value = $storage->get_metadata($folder, $this->expire_annotation);
 
         if (is_array($value)) {
-            return ($value[$folder] ?? false) ? intval($value[$folder][$this->expire_annotation]) : 0;
+            return !empty($value[$folder]) ? intval($value[$folder][$this->expire_annotation] ?? 0) : 0;
         }
 
         return false;
diff --git a/plugins/kolab_notes/kolab_notes.php b/plugins/kolab_notes/kolab_notes.php
index a73d17a4..b0674f86 100644
--- a/plugins/kolab_notes/kolab_notes.php
+++ b/plugins/kolab_notes/kolab_notes.php
@@ -540,7 +540,7 @@ class kolab_notes extends rcube_plugin
 
         // deliver from in-memory cache
         $key = $list_id . ':' . $uid;
-        if ($this->cache[$key] ?? false) {
+        if (!empty($this->cache[$key])) {
             return $this->cache[$key];
         }
 
@@ -732,7 +732,7 @@ class kolab_notes extends rcube_plugin
             return false;
 
         // moved from another folder
-        if (($note['_fromlist'] ?? false) && ($fromfolder = $this->get_folder($note['_fromlist']))) {
+        if (!empty($note['_fromlist']) && ($fromfolder = $this->get_folder($note['_fromlist']))) {
             if (!$fromfolder->move($note['uid'], $folder->name))
                 return false;
 
@@ -741,7 +741,7 @@ class kolab_notes extends rcube_plugin
 
         // load previous version of this record to merge
         $old = null;
-        if ($note['uid']) {
+        if (!empty($note['uid'])) {
             $old = $folder->get_object($note['uid']);
             if (!$old || PEAR::isError($old))
                 return false;
@@ -1407,7 +1407,7 @@ class kolab_notes extends rcube_plugin
         }
 
         // make list of categories unique
-        if (is_array($object['tags'] ?? null)) {
+        if (!empty($object['tags'])) {
             $object['tags'] = array_unique(array_filter($object['tags']));
         }
 
diff --git a/plugins/kolab_notes/kolab_notes_ui.php b/plugins/kolab_notes/kolab_notes_ui.php
index 2d3ad12d..855f3352 100644
--- a/plugins/kolab_notes/kolab_notes_ui.php
+++ b/plugins/kolab_notes/kolab_notes_ui.php
@@ -76,13 +76,14 @@ class kolab_notes_ui
     public function folders($attrib)
     {
         $attrib += array('id' => 'rcmkolabnotebooks');
+        $is_select = ($attrib['type'] ?? null) == 'select';
 
-        if (($attrib['type'] ?? null) == 'select') {
+        if ($is_select) {
             $attrib['is_escaped'] = true;
             $select = new html_select($attrib);
         }
 
-        $tree = ($attrib['type'] ?? null) != 'select' ? true : null;
+        $tree  = $is_select ? true : null;
         $lists = $this->plugin->get_lists($tree);
         $jsenv = array();
 
@@ -94,12 +95,12 @@ class kolab_notes_ui
             foreach ($lists as $prop) {
                 $id = $prop['id'];
 
-                if (!$prop['virtual']) {
+                if (empty($prop['virtual'])) {
                     unset($prop['user_id']);
                     $jsenv[$id] = $prop;
                 }
 
-                if ($attrib['type'] == 'select') {
+                if ($is_select) {
                     if ($prop['editable'] || strpos($prop['rights'], 'i') !== false) {
                         $select->add($prop['name'], $prop['id']);
                     }
@@ -115,7 +116,7 @@ class kolab_notes_ui
         $this->rc->output->set_env('kolab_notebooks', $jsenv);
         $this->rc->output->add_gui_object('notebooks', $attrib['id']);
 
-        return ($attrib['type'] ?? null) == 'select' ? $select->show() : html::tag('ul', $attrib, $html, html::$common_attrib);
+        return $is_select ? $select->show() : html::tag('ul', $attrib, $html, html::$common_attrib);
     }
 
     /**
@@ -139,7 +140,7 @@ class kolab_notes_ui
             if (strlen($content)) {
                 $out .= html::tag('li', array(
                       'id' => 'rcmliknb' . rcube_utils::html_identifier($id),
-                      'class' => $prop['group'] . (($prop['virtual'] ?? false) ? ' virtual' : ''),
+                      'class' => $prop['group'] . (!empty($prop['virtual']) ? ' virtual' : ''),
                     ),
                     $content);
             }
@@ -153,13 +154,13 @@ class kolab_notes_ui
      */
     public function folder_list_item($id, $prop, &$jsenv, $checkbox = false)
     {
-        if (!($prop['virtual'] ?? false)) {
+        if (empty($prop['virtual'])) {
             unset($prop['user_id']);
             $jsenv[$id] = $prop;
         }
 
         $classes = array('folder');
-        if ($prop['virtual'] ?? false) {
+        if (!empty($prop['virtual'])) {
             $classes[] = 'virtual';
         }
         else if (!$prop['editable']) {
@@ -172,21 +173,21 @@ class kolab_notes_ui
             $classes[] = $prop['class'];
         }
 
-        $title = $prop['title'] ?? ($prop['name'] != $prop['listname'] || strlen($prop['name']) > 25 ?
+        $title = !empty($prop['title']) ? $prop['title'] : ($prop['name'] != $prop['listname'] || strlen($prop['name']) > 25 ?
           html_entity_decode($prop['name'], ENT_COMPAT, RCUBE_CHARSET) : '');
 
         $label_id = 'nl:' . $id;
-        $attr = ($prop['virtual'] ?? false) ? array('tabindex' => '0') : array('href' => $this->rc->url(array('_list' => $id)));
+        $attr = !empty($prop['virtual']) ? array('tabindex' => '0') : array('href' => $this->rc->url(array('_list' => $id)));
         return html::div(join(' ', $classes),
             html::a($attr + array('class' => 'listname', 'title' => $title, 'id' => $label_id), $prop['listname'] ?: $prop['name']) .
-            (($prop['virtual'] ?? false) ? '' :
+            (!empty($prop['virtual']) ? '' :
                 ($checkbox ?
                     html::tag('input', array('type' => 'checkbox', 'name' => '_list[]', 'value' => $id, 'checked' => $prop['active'], 'aria-labelledby' => $label_id)) :
                     ''
                 ) .
                 html::span('handle', '') .
                 html::span('actions',
-                    (!$prop['default'] ?
+                    (empty($prop['default']) ?
                         html::a(array('href' => '#', 'class' => 'remove', 'title' => $this->plugin->gettext('removelist')), ' ') :
                         ''
                     ) .
diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index 8e8194c0..efd7a7be 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -417,7 +417,7 @@ abstract class kolab_format
     protected function update_uid()
     {
         // get generated UID
-        if (!($this->data['uid'] ?? null)) {
+        if (empty($this->data['uid'])) {
             if ($this->xmlobject) {
                 $this->data['uid'] = $this->xmlobject->getSerializedUID();
             }
@@ -541,13 +541,14 @@ abstract class kolab_format
     {
         $this->init();
 
-        if (!empty($object['uid']))
+        if (!empty($object['uid'])) {
             $this->obj->setUid($object['uid']);
+        }
 
         // set some automatic values if missing
         if (method_exists($this->obj, 'setCreated')) {
             // Always set created date to workaround libkolabxml (>1.1.4) bug
-            $created = $object['created'] ?? new DateTime('now');
+            $created = !empty($object['created']) ? $object['created'] : new DateTime('now');
             $created->setTimezone(new DateTimeZone('UTC')); // must be UTC
             $this->obj->setCreated(self::get_datetime($created));
             $object['created'] = $created;
diff --git a/plugins/libkolab/lib/kolab_format_configuration.php b/plugins/libkolab/lib/kolab_format_configuration.php
index 7d7bba4a..c0979c25 100644
--- a/plugins/libkolab/lib/kolab_format_configuration.php
+++ b/plugins/libkolab/lib/kolab_format_configuration.php
@@ -80,19 +80,19 @@ class kolab_format_configuration extends kolab_format
         case 'relation':
             $relation = new Relation(strval($object['name']), strval($object['category']));
 
-            if ($object['color'] ?? false) {
+            if (!empty($object['color'])) {
                 $relation->setColor($object['color']);
             }
-            if ($object['parent'] ?? false) {
+            if (!empty($object['parent'])) {
                 $relation->setParent($object['parent']);
             }
-            if ($object['iconName'] ?? false) {
+            if (!empty($object['iconName'])) {
                 $relation->setIconName($object['iconName']);
             }
-            if (($object['priority'] ?? 0) > 0) {
+            if (!empty($object['priority'])) {
                 $relation->setPriority((int) $object['priority']);
             }
-            if (!empty($object['members'] ?? null)) {
+            if (!empty($object['members'])) {
                 $relation->setMembers(self::array2vector($object['members']));
             }
 
@@ -106,7 +106,7 @@ class kolab_format_configuration extends kolab_format
             foreach ((array)($object['snippets'] ?? []) as $item) {
                 $snippet = new snippet($item['name'], $item['text']);
                 $snippet->setTextType(strtolower($item['type']) == 'html' ? Snippet::HTML : Snippet::Plain);
-                if ($item['shortcut'] ?? false) {
+                if (!empty($item['shortcut'])) {
                     $snippet->setShortCut($item['shortcut']);
                 }
 
diff --git a/plugins/libkolab/lib/kolab_format_contact.php b/plugins/libkolab/lib/kolab_format_contact.php
index ac5be839..bf82aa03 100644
--- a/plugins/libkolab/lib/kolab_format_contact.php
+++ b/plugins/libkolab/lib/kolab_format_contact.php
@@ -113,16 +113,16 @@ class kolab_format_contact extends kolab_format
         // organisation related properties (affiliation)
         $org = new Affiliation;
         $offices = new vectoraddress;
-        if ($object['organization'] ?? null)
+        if (!empty($object['organization']))
             $org->setOrganisation($object['organization']);
-        if ($object['department'] ?? null)
+        if (!empty($object['department']))
             $org->setOrganisationalUnits(self::array2vector($object['department']));
-        if ($object['profession'] ?? null)
+        if (!empty($object['profession']))
             $org->setRoles(self::array2vector($object['profession']));
 
         $rels = new vectorrelated;
         foreach (array('manager','assistant') as $field) {
-            if (!empty($object[$field] ?? null)) {
+            if (!empty($object[$field])) {
                 $reltype = $this->relatedmap[$field];
                 foreach ((array)$object[$field] as $value) {
                     $rels->push(new Related(Related::Text, $value, $reltype));
@@ -160,17 +160,17 @@ class kolab_format_contact extends kolab_format
             $type = $this->addresstypes[$address['type']];
             if (isset($type))
                 $adr->setTypes($type);
-            else if ($address['type'] ?? null)
+            else if (!empty($address['type']))
                 $adr->setLabel($address['type']);
-            if ($address['street'] ?? null)
+            if (!empty($address['street']))
                 $adr->setStreet($address['street']);
-            if ($address['locality'] ?? null)
+            if (!empty($address['locality']))
                 $adr->setLocality($address['locality']);
-            if ($address['code'] ?? null)
+            if (!empty($address['code']))
                 $adr->setCode($address['code']);
-            if ($address['region'] ?? null)
+            if (!empty($address['region']))
                 $adr->setRegion($address['region']);
-            if ($address['country'] ?? null)
+            if (!empty($address['country']))
                 $adr->setCountry($address['country']);
 
             if (($address['type'] ?? null) == 'office')
@@ -210,7 +210,7 @@ class kolab_format_contact extends kolab_format
         if (isset($object['anniversary']))
             $this->obj->setAnniversary(self::get_datetime($object['anniversary'], false, true));
 
-        if (!empty($object['photo'] ?? null)) {
+        if (!empty($object['photo'])) {
             if ($type = rcube_mime::image_content_type($object['photo']))
                 $this->obj->setPhoto($object['photo'], $type);
         }
@@ -230,7 +230,7 @@ class kolab_format_contact extends kolab_format
             }
         }
         // add other relateds
-        if (is_array($object['related'] ?? null)) {
+        if (!empty($object['related']) && is_array($object['related'])) {
             foreach ($object['related'] as $value) {
                 $rels->push(new Related(Related::Text, $value));
             }
@@ -248,8 +248,8 @@ class kolab_format_contact extends kolab_format
                 $pkcs7_index = $i;
         }
 
-        $pgpkey   = ($object['pgppublickey'] ?? false)   ? new Key($object['pgppublickey'], Key::PGP) : new Key();
-        $pkcs7key = ($object['pkcs7publickey'] ?? false) ? new Key($object['pkcs7publickey'], Key::PKCS7_MIME) : new Key();
+        $pgpkey   = !empty($object['pgppublickey']) ? new Key($object['pgppublickey'], Key::PGP) : new Key();
+        $pkcs7key = !empty($object['pkcs7publickey']) ? new Key($object['pkcs7publickey'], Key::PKCS7_MIME) : new Key();
 
         if ($pgp_index >= 0)
             $keys->set($pgp_index, $pgpkey);
diff --git a/plugins/libkolab/lib/kolab_format_event.php b/plugins/libkolab/lib/kolab_format_event.php
index ad5e8b71..d9d4127f 100644
--- a/plugins/libkolab/lib/kolab_format_event.php
+++ b/plugins/libkolab/lib/kolab_format_event.php
@@ -67,18 +67,18 @@ class kolab_format_event extends kolab_format_xcal
         $status = kolabformat::StatusUndefined;
         if ($object['free_busy'] == 'tentative')
             $status = kolabformat::StatusTentative;
-        if ($object['cancelled'] ?? false)
+        if (!empty($object['cancelled']))
             $status = kolabformat::StatusCancelled;
-        else if ($object['status'] && array_key_exists($object['status'], $this->status_map))
+        else if (!empty($object['status']) && array_key_exists($object['status'], $this->status_map))
             $status = $this->status_map[$object['status']];
         $this->obj->setStatus($status);
 
         // save (recurrence) exceptions
-        if (is_array($object['recurrence'] ?? null) && is_array($object['recurrence']['EXCEPTIONS'] ?? null) && !isset($object['exceptions'])) {
+        if (!empty($object['recurrence']['EXCEPTIONS']) && !isset($object['exceptions'])) {
             $object['exceptions'] = $object['recurrence']['EXCEPTIONS'];
         }
 
-        if (is_array($object['exceptions'] ?? null)) {
+        if (!empty($object['exceptions'])) {
             $recurrence_id_format = libkolab::recurrence_id_format($object);
             $vexceptions = new vectorevent;
             foreach ($object['exceptions'] as $i => $exception) {
@@ -112,12 +112,13 @@ class kolab_format_event extends kolab_format_xcal
             }
         }
 
-        if (($object['recurrence_date'] ?? false) && $object['recurrence_date'] instanceof DateTimeInterface) {
-            if ($object['recurrence'] ?? false) {
+        if (!empty($object['recurrence_date']) && $object['recurrence_date'] instanceof DateTimeInterface) {
+            if (!empty($object['recurrence'])) {
                 // unset recurrence_date for master events with rrule
                 $object['recurrence_date'] = null;
             }
-            $this->obj->setRecurrenceID(self::get_datetime($object['recurrence_date'], null, $object['allday']), (bool)$object['thisandfuture']);
+            $recid = self::get_datetime($object['recurrence_date'], null, !empty($object['allday']));
+            $this->obj->setRecurrenceID($recid, !empty($object['thisandfuture']));
         }
 
         // cache this data
@@ -216,7 +217,7 @@ class kolab_format_event extends kolab_format_xcal
             $object['exceptions'] = $recurrence_exceptions;
 
             // also link with recurrence.EXCEPTIONS for compatibility
-            if (is_array($object['recurrence'] ?? null)) {
+            if (!empty($object['recurrence']) && isset($object['exceptions'])) {
                 $object['recurrence']['EXCEPTIONS'] = &$object['exceptions'];
             }
         }
diff --git a/plugins/libkolab/lib/kolab_format_file.php b/plugins/libkolab/lib/kolab_format_file.php
index ba182d3c..b4739b61 100644
--- a/plugins/libkolab/lib/kolab_format_file.php
+++ b/plugins/libkolab/lib/kolab_format_file.php
@@ -48,7 +48,7 @@ class kolab_format_file extends kolab_format
         }
 
         // Add file attachment
-        if (!empty($object['_attachments'] ?? null)) {
+        if (!empty($object['_attachments'])) {
             $cid         = key($object['_attachments']);
             $attach_attr = $object['_attachments'][$cid];
             $attach      = new Attachment;
@@ -98,7 +98,7 @@ class kolab_format_file extends kolab_format
      *
      * @return array  Config object data as hash array
      */
-    public function to_array($data = array())
+    public function to_array($data = [])
     {
         // return cached result
         if (!empty($this->data)) {
@@ -109,10 +109,10 @@ class kolab_format_file extends kolab_format
         $object = parent::to_array($data);
 
         // read object properties
-        $object += array(
+        $object += [
             'categories'  => self::vector2array($this->obj->categories()),
             'notes'       => $this->obj->note(),
-        );
+        ];
 
         return $this->data = $object;
     }
@@ -124,19 +124,17 @@ class kolab_format_file extends kolab_format
      */
     public function get_tags()
     {
-        $tags = array();
+        $tags = [];
 
-        foreach ((array)($this->data['categories'] ?? null) as $cat) {
+        foreach ((array)($this->data['categories'] ?? []) as $cat) {
             $tags[] = rcube_utils::normalize_string($cat);
         }
 
         // Add file mimetype to tags
-        if (!empty($this->data['_attachments'] ?? null)) {
-            reset($this->data['_attachments']);
-            $key        = key($this->data['_attachments']);
-            $attachment = $this->data['_attachments'][$key];
+        if (!empty($this->data['_attachments'])) {
+            $attachment = $this->data['_attachments'][array_key_first($this->data['_attachments'])];
 
-            if ($attachment['mimetype'] ?? false) {
+            if (!empty($attachment['mimetype'])) {
                 $tags[] = $attachment['mimetype'];
             }
         }
diff --git a/plugins/libkolab/lib/kolab_format_note.php b/plugins/libkolab/lib/kolab_format_note.php
index b279eb23..e1ba16fa 100644
--- a/plugins/libkolab/lib/kolab_format_note.php
+++ b/plugins/libkolab/lib/kolab_format_note.php
@@ -124,20 +124,24 @@ class kolab_format_note extends kolab_format
     {
         $data = '';
         foreach (self::$fulltext_cols as $col) {
+            if (empty($this->data[$col])) {
+                continue;
+            }
+
             // convert HTML content to plain text
             if ($col == 'description' && preg_match('/<(html|body)(\s[a-z]|>)/', $this->data[$col], $m) && strpos($this->data[$col], '</'.$m[1].'>')) {
-                $converter = new rcube_html2text($this->data[$col] ?? null, false, false, 0);
+                $converter = new rcube_html2text($this->data[$col], false, false, 0);
                 $val = $converter->get_text();
             }
             else {
-                $val = is_array($this->data[$col] ?? null) ? join(' ', $this->data[$col] ?? null) : ($this->data[$col] ?? null);
+                $val = is_array($this->data[$col]) ? join(' ', $this->data[$col]) : $this->data[$col];
             }
 
-            if (strlen($val))
+            if (strlen($val)) {
                 $data .= $val . ' ';
+            }
         }
 
         return array_filter(array_unique(rcube_utils::normalize_string($data, true)));
     }
-
 }
diff --git a/plugins/libkolab/lib/kolab_format_task.php b/plugins/libkolab/lib/kolab_format_task.php
index 25fc6ace..398c8da4 100644
--- a/plugins/libkolab/lib/kolab_format_task.php
+++ b/plugins/libkolab/lib/kolab_format_task.php
@@ -58,12 +58,16 @@ class kolab_format_task extends kolab_format_xcal
         $status = kolabformat::StatusUndefined;
         if ($object['complete'] == 100 && !array_key_exists('status', $object))
             $status = kolabformat::StatusCompleted;
-        else if ($object['status'] && array_key_exists($object['status'], $this->status_map))
+        else if (!empty($object['status']) && array_key_exists($object['status'], $this->status_map))
             $status = $this->status_map[$object['status']];
         $this->obj->setStatus($status);
 
-        $this->obj->setStart(self::get_datetime($object['start'] ?? null, null, ($object['start'] ?? null) ? $object['start']->_dateonly : null));
-        $this->obj->setDue(self::get_datetime($object['due'] ?? null, null, ($object['due'] ?? null) ? $object['due']->_dateonly : null));
+        if (!empty($object['start'])) {
+            $this->obj->setStart(self::get_datetime($object['start'], null, !empty($object['start']->_dateonly)));
+        }
+        if (!empty($object['due'])) {
+            $this->obj->setDue(self::get_datetime($object['due'], null, !empty($object['due']->_dateonly)));
+        }
 
         $related = new vectors;
         if (!empty($object['parent_id']))
@@ -76,7 +80,7 @@ class kolab_format_task extends kolab_format_xcal
     }
 
     /**
-     *
+     * Check if the object is valid
      */
     public function is_valid()
     {
@@ -123,7 +127,7 @@ class kolab_format_task extends kolab_format_xcal
      */
     public function get_reference_date()
     {
-        if ($this->data['due'] && $this->data['due'] instanceof DateTimeInterface) {
+        if (!empty($this->data['due']) && $this->data['due'] instanceof DateTimeInterface) {
             return $this->data['due'];
         }
 
@@ -143,10 +147,10 @@ class kolab_format_task extends kolab_format_xcal
         if (($object['status'] ?? null) == 'COMPLETED' || (($object['complete'] ?? null) == 100 && empty($object['status'] ?? null)))
             $tags[] = 'x-complete';
 
-        if (($object['priority'] ?? 0) == 1)
+        if (!empty($object['priority']))
             $tags[] = 'x-flagged';
 
-        if ($object['parent_id'] ?? false)
+        if (!empty($object['parent_id']))
             $tags[] = 'x-parent:' . $object['parent_id'];
 
         return array_unique($tags);
diff --git a/plugins/libkolab/lib/kolab_format_xcal.php b/plugins/libkolab/lib/kolab_format_xcal.php
index b8b4513f..2ddeff63 100644
--- a/plugins/libkolab/lib/kolab_format_xcal.php
+++ b/plugins/libkolab/lib/kolab_format_xcal.php
@@ -420,7 +420,7 @@ abstract class kolab_format_xcal extends kolab_format
         $rr = new RecurrenceRule;
         $rr->setFrequency(RecurrenceRule::FreqNone);
 
-        if (($object['recurrence'] ?? null) && !empty($object['recurrence']['FREQ'])) {
+        if (!empty($object['recurrence']['FREQ'])) {
             $freq     = $object['recurrence']['FREQ'];
             $bysetpos = explode(',', $object['recurrence']['BYSETPOS']);
 
@@ -502,7 +502,7 @@ abstract class kolab_format_xcal extends kolab_format
         // save alarm(s)
         $valarms = new vectoralarm;
         $valarm_hashes = array();
-        if ($object['valarms'] ?? null) {
+        if (!empty($object['valarms'])) {
             foreach ($object['valarms'] as $valarm) {
                 if (!array_key_exists($valarm['action'], $this->alarm_type_map)) {
                     continue;  // skip unknown alarm types
@@ -577,7 +577,7 @@ abstract class kolab_format_xcal extends kolab_format
             }
         }
         // legacy support
-        else if ($object['alarms'] ?? null) {
+        else if (!empty($object['alarms'])) {
             list($offset, $type) = explode(":", $object['alarms']);
 
             if ($type == 'EMAIL' && !empty($object['_owner'])) {  // email alarms implicitly go to event owner
@@ -638,14 +638,18 @@ abstract class kolab_format_xcal extends kolab_format
         foreach (self::$fulltext_cols as $colname) {
             list($col, $field) = array_pad(explode(':', $colname), 2, null);
 
+            if (empty($object[$col])) {
+                continue;
+            }
+
             if ($field) {
                 $a = array();
-                foreach ((array)($object[$col] ?? []) as $attr)
+                foreach ((array) $object[$col] as $attr)
                     $a[] = $attr[$field];
                 $val = join(' ', $a);
             }
             else {
-                $val = is_array($object[$col] ?? null) ? join(' ', $object[$col]) : $object[$col] ?? null;
+                $val = is_array($object[$col]) ? join(' ', $object[$col]) : $object[$col];
             }
 
             if (strlen($val))
@@ -655,7 +659,7 @@ abstract class kolab_format_xcal extends kolab_format
         $words = rcube_utils::normalize_string($data, true);
 
         // collect words from recurrence exceptions
-        if (is_array($object['exceptions'] ?? null)) {
+        if (!empty($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 $words = array_merge($words, $this->get_words($exception));
             }
@@ -674,12 +678,12 @@ abstract class kolab_format_xcal extends kolab_format
         $tags = array();
         $object = $obj ?: $this->data;
 
-        if (!empty($object['valarms'] ?? null)) {
+        if (!empty($object['valarms'])) {
             $tags[] = 'x-has-alarms';
         }
 
         // create tags reflecting participant status
-        if (is_array($object['attendees'] ?? null)) {
+        if (!empty($object['attendees'])) {
             foreach ($object['attendees'] as $attendee) {
                 if (!empty($attendee['email']) && !empty($attendee['status']))
                     $tags[] = 'x-partstat:' . $attendee['email'] . ':' . strtolower($attendee['status']);
@@ -687,14 +691,14 @@ abstract class kolab_format_xcal extends kolab_format
         }
 
         // collect tags from recurrence exceptions
-        if (is_array($object['exceptions'] ?? null)) {
+        if (!empty($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 $tags = array_merge($tags, $this->get_tags($exception));
             }
         }
 
         if (!empty($object['status'])) {
-          $tags[] = 'x-status:' . strtolower($object['status']);
+            $tags[] = 'x-status:' . strtolower($object['status']);
         }
 
         return array_unique($tags);
@@ -713,14 +717,14 @@ abstract class kolab_format_xcal extends kolab_format
         $reschedule = false;
 
         if (!is_array($old)) {
-            $old = $this->data['uid'] ? $this->data : $this->to_array();
+            $old = !empty($this->data['uid']) ? $this->data : $this->to_array();
         }
 
         foreach ($this->_scheduling_properties ?: self::$scheduling_properties as $prop) {
             $a = $old[$prop] ?? null;
             $b = $object[$prop] ?? null;
 
-            if (($object['allday'] ?? false)
+            if (!empty($object['allday'])
                 && ($prop == 'start' || $prop == 'end')
                 && $a instanceof DateTimeInterface
                 && $b instanceof DateTimeInterface
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 13db9b5b..1ef2138e 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -129,7 +129,7 @@ class kolab_storage
             $config = self::$config->get($name);
         }
 
-        if (self::$ldap[$name] ?? false) {
+        if (!empty(self::$ldap[$name])) {
             return self::$ldap[$name];
         }
 
diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index 22e9140c..a559d7cf 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -1079,11 +1079,11 @@ class kolab_storage_cache
      */
     protected function _unserialize($sql_arr)
     {
-        if (($sql_arr['fast-mode'] ?? false) && !empty($sql_arr['data']) && ($object = json_decode($sql_arr['data'], true))) {
+        if (!empty($sql_arr['fast-mode']) && !empty($sql_arr['data']) && ($object = json_decode($sql_arr['data'], true))) {
             $object['uid'] = $sql_arr['uid'];
 
             foreach ($this->data_props as $prop) {
-                if (isset($object[$prop]) && is_array($object[$prop]) && isset($object[$prop]['cl']) && $object[$prop]['cl'] == 'DateTime') {
+                if (!empty($object[$prop]['cl']) && $object[$prop]['cl'] == 'DateTime') {
                     $object[$prop] = new DateTime($object[$prop]['dt'], new DateTimeZone($object[$prop]['tz']));
                 }
                 else if (!isset($object[$prop]) && isset($sql_arr[$prop])) {
@@ -1099,9 +1099,9 @@ class kolab_storage_cache
                 $object['changed'] = new DateTime($sql_arr['changed']);
             }
 
-            $object['_type']     = $sql_arr['type'] ?? $this->folder->type;
-            $object['_msguid']   = $sql_arr['msguid'];
-            $object['_mailbox']  = $this->folder->name;
+            $object['_type']    = !empty($sql_arr['type']) ? $sql_arr['type'] : $this->folder->type;
+            $object['_msguid']  = $sql_arr['msguid'];
+            $object['_mailbox'] = $this->folder->name;
         }
         // Fetch object xml
         else {
diff --git a/plugins/libkolab/lib/kolab_storage_cache_contact.php b/plugins/libkolab/lib/kolab_storage_cache_contact.php
index ab39ef7f..754bdd3e 100644
--- a/plugins/libkolab/lib/kolab_storage_cache_contact.php
+++ b/plugins/libkolab/lib/kolab_storage_cache_contact.php
@@ -37,22 +37,26 @@ class kolab_storage_cache_contact extends kolab_storage_cache
         $sql_data = parent::_serialize($object);
         $sql_data['type'] = $object['_type'];
 
-        // columns for sorting
-        $sql_data['name']      = rcube_charset::clean(($object['name'] ?? null) . ($object['prefix'] ?? null));
-        $sql_data['firstname'] = rcube_charset::clean(($object['firstname'] ?? null) . ($object['middlename'] ?? null) . ($object['surname'] ?? null));
-        $sql_data['surname']   = rcube_charset::clean(($object['surname'] ?? null)   . ($object['firstname'] ?? null)  . ($object['middlename'] ?? null));
-        $sql_data['email']     = rcube_charset::clean(is_array($object['email'] ?? null) ? $object['email'][0] : ($object['email'] ?? null));
+        $email = is_array($object['email'] ?? null) ? $object['email'][0] : ($object['email'] ?? '');
 
-        if (is_array($sql_data['email'] ?? null)) {
-            $sql_data['email'] = $sql_data['email']['address'];
+        // columns for sorting
+        $sql_data['name']      = rcube_charset::clean(($object['name'] ?? '') . ($object['prefix'] ?? ''));
+        $sql_data['firstname'] = rcube_charset::clean(($object['firstname'] ?? '') . ($object['middlename'] ?? '') . ($object['surname'] ?? ''));
+        $sql_data['surname']   = rcube_charset::clean(($object['surname'] ?? '')   . ($object['firstname'] ?? '')  . ($object['middlename'] ?? ''));
+
+        if (is_array($email)) {
+            $sql_data['email'] = $email['address'] ?? '';
+        }
+        else {
+            $sql_data['email'] = rcube_charset::clean($email);
         }
         // avoid value being null
-        if (empty($sql_data['email'] ?? null)) {
+        if (empty($sql_data['email'])) {
             $sql_data['email'] = '';
         }
 
         // use organization if name is empty
-        if (empty($sql_data['name'] ?? null) && !empty($object['organization'] ?? null)) {
+        if (empty($sql_data['name']) && !empty($object['organization'])) {
             $sql_data['name'] = rcube_charset::clean($object['organization']);
         }
 
diff --git a/plugins/libkolab/lib/kolab_storage_cache_event.php b/plugins/libkolab/lib/kolab_storage_cache_event.php
index 625b10fe..94b81f17 100644
--- a/plugins/libkolab/lib/kolab_storage_cache_event.php
+++ b/plugins/libkolab/lib/kolab_storage_cache_event.php
@@ -39,14 +39,14 @@ class kolab_storage_cache_event extends kolab_storage_cache
         $sql_data['dtend']   = $this->_convert_datetime($object['end'] ?? null);
 
         // extend date range for recurring events
-        if ($object['recurrence'] ?? null) {
+        if (!empty($object['recurrence'])) {
             $recurrence = new kolab_date_recurrence($object['_formatobj']);
             $dtend = $recurrence->end() ?: new DateTime('now +100 years');
             $sql_data['dtend'] = $this->_convert_datetime($dtend);
         }
 
         // extend start/end dates to spawn all exceptions
-        if (is_array($object['exceptions'] ?? null)) {
+        if (!empty($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 if (($exception['start'] ?? null) instanceof DateTimeInterface) {
                     $exstart = $this->_convert_datetime($exception['start']);
diff --git a/plugins/libkolab/lib/kolab_storage_config.php b/plugins/libkolab/lib/kolab_storage_config.php
index 64d305ab..76ffaf30 100644
--- a/plugins/libkolab/lib/kolab_storage_config.php
+++ b/plugins/libkolab/lib/kolab_storage_config.php
@@ -256,7 +256,7 @@ class kolab_storage_config
         }
 
         // find folder object
-        if ($object['_mailbox'] ?? false) {
+        if (!empty($object['_mailbox'])) {
             foreach ($this->folders as $folder) {
                 if ($folder->name == $object['_mailbox']) {
                     break;
@@ -676,9 +676,9 @@ class kolab_storage_config
 
             // assign links to objects
             foreach ((array) $relation['members'] as $member) {
-                if (($id = ($ids[$member] ?? null)) !== null) {
-                    foreach ($id as $i) {
-                        $records[$i]['links'] = array_unique(array_merge($records[$i]['links'], $members));
+                if (!empty($ids[$member])) {
+                    foreach ($ids[$member] as $i) {
+                        $records[$i]['links'] = array_unique(array_merge($records[$i]['links'] ?? [], $members));
                     }
                 }
             }
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index 6cf7d1b8..56abbf21 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -197,7 +197,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
     {
         $fdata = $this->get_imap_data();
         $this->check_error();
-        return sprintf('%d-%d-%d', $fdata['UIDVALIDITY'] ?? null, $fdata['HIGHESTMODSEQ'] ?? null, $fdata['UIDNEXT'] ?? null);
+        return sprintf('%d-%d-%d', $fdata['UIDVALIDITY'] ?? 0, $fdata['HIGHESTMODSEQ'] ?? 0, $fdata['UIDNEXT'] ?? 0);
     }
 
     /**
@@ -637,7 +637,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
         }
 
         // process attachments
-        if (is_array($object['_attachments'] ?? null)) {
+        if (!empty($object['_attachments'])) {
             $numatt = count($object['_attachments']);
             foreach ($object['_attachments'] as $key => $attachment) {
                 // FIXME: kolab_storage and Roundcube attachment hooks use different fields!
@@ -922,7 +922,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
         $format = null;
         if (is_object($object['_formatobj'] ?? null))
             $format = $object['_formatobj'];
-        else if ($object['_msguid'] ?? null && ($old = $this->cache->get($object['_msguid'], $type, $object['_mailbox'] ?? null)))
+        else if (!empty($object['_msguid']) && ($old = $this->cache->get($object['_msguid'], $type, $object['_mailbox'] ?? null)))
             $format = $old['_formatobj'] ?? null;
 
         // create new kolab_format instance
diff --git a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
index b1eea3e0..8f65ffdc 100644
--- a/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
+++ b/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php
@@ -653,11 +653,11 @@ class tasklist_kolab_driver extends tasklist_driver
             }
         }
 
-        if ($filter['since'] ?? false) {
+        if (!empty($filter['since'])) {
             $query[] = array('changed', '>=', $filter['since']);
         }
 
-        if ($filter['uid'] ?? false) {
+        if (!empty($filter['uid'])) {
             $query[] = array('uid', '=', (array) $filter['uid']);
         }
 
@@ -709,7 +709,7 @@ class tasklist_kolab_driver extends tasklist_driver
                 $folder = $this->folders[$list_id];
             if (is_numeric($list_id) || !$folder)
                 continue;
-            if (!($this->tasks[$id] ?? false) && ($object = $folder->get_object($id))) {
+            if (empty($this->tasks[$id]) && ($object = $folder->get_object($id))) {
                 $this->load_tags($object);
                 $this->tasks[$id] = $this->_to_rcube_task($object, $list_id);
                 break;
@@ -1302,10 +1302,10 @@ class tasklist_kolab_driver extends tasklist_driver
             $task['created'] = $record['created'];
         }
 
-        if ($record['valarms'] ?? false) {
+        if (!empty($record['valarms'])) {
             $task['valarms'] = $record['valarms'];
         }
-        else if ($record['alarms'] ?? false) {
+        else if (!empty($record['alarms'])) {
             $task['alarms'] = $record['alarms'];
         }
 
@@ -1320,7 +1320,7 @@ class tasklist_kolab_driver extends tasklist_driver
             }
         }
 
-        if (!empty($record['_attachments'] ?? [])) {
+        if (!empty($record['_attachments'])) {
             foreach ($record['_attachments'] as $key => $attachment) {
                 if ($attachment !== false) {
                     if (empty($attachment['name'])) {
@@ -1381,7 +1381,7 @@ class tasklist_kolab_driver extends tasklist_driver
         if ($task['complete'] == 1.0 && empty($task['complete']))
             $object['status'] = 'COMPLETED';
 
-        if ($task['flagged'] ?? false)
+        if (!empty($task['flagged']))
             $object['priority'] = 1;
         else
             $object['priority'] = ($old['priority'] ?? 0) > 1 ? $old['priority'] : 0;
@@ -1462,7 +1462,7 @@ class tasklist_kolab_driver extends tasklist_driver
 
         // load previous version of this task to merge
         $old = null;
-        if ($task['id'] ?? null) {
+        if (!empty($task['id'])) {
             $old = $folder->get_object($task['uid']);
             if (!$old || PEAR::isError($old))
                 return false;
diff --git a/plugins/tasklist/tasklist_ui.php b/plugins/tasklist/tasklist_ui.php
index 6d36149c..24d388ba 100644
--- a/plugins/tasklist/tasklist_ui.php
+++ b/plugins/tasklist/tasklist_ui.php
@@ -45,14 +45,14 @@ class tasklist_ui
         }
 
         // add taskbar button
-        $this->plugin->add_button(array(
+        $this->plugin->add_button([
             'command'    => 'tasks',
             'class'      => 'button-tasklist',
             'classsel'   => 'button-tasklist button-selected',
             'innerclass' => 'button-inner',
             'label'      => 'tasklist.navtitle',
             'type'       => 'link'
-        ), 'taskbar');
+        ], 'taskbar');
 
         $this->plugin->include_stylesheet($this->plugin->local_skin_path() . '/tasklist.css');
 
@@ -92,11 +92,11 @@ class tasklist_ui
         }
 
         $identity['emails'][] = $this->rc->user->get_username();
-        $settings['identity'] = array(
+        $settings['identity'] = [
             'name'   => $identity['name'],
             'email'  => strtolower($identity['email']),
             'emails' => ';' . strtolower(join(';', $identity['emails']))
-        );
+        ];
 
         if ($list = rcube_utils::get_input_value('_list', rcube_utils::INPUT_GPC)) {
             $settings['selected_list'] = $list;
@@ -218,18 +218,18 @@ class tasklist_ui
             $prop = $data[$id];
             $is_collapsed = false; // TODO: determine this somehow?
 
-            $content = $this->tasklist_list_item($id, $prop, $jsenv, $attrib['activeonly'] ?? null);
+            $content = $this->tasklist_list_item($id, $prop, $jsenv, !empty($attrib['activeonly']));
 
             if (!empty($folder->children)) {
-                $content .= html::tag('ul', array('style' => ($is_collapsed ? "display:none;" : null)),
+                $content .= html::tag('ul', ['style' => ($is_collapsed ? "display:none;" : null)],
                     $this->list_tree_html($folder, $data, $jsenv, $attrib));
             }
 
             if (strlen($content)) {
-                $out .= html::tag('li', array(
+                $out .= html::tag('li', [
                       'id' => 'rcmlitasklist' . rcube_utils::html_identifier($id),
                       'class' => $prop['group'] . ($prop['virtual'] ? ' virtual' : ''),
-                    ),
+                    ],
                     $content);
             }
         }
@@ -277,13 +277,13 @@ class tasklist_ui
             $classes[] = $prop['class'];
         }
 
-        if (!$activeonly || $prop['active']) {
+        if (!$activeonly || !empty($prop['active'])) {
             $label_id = 'tl:' . $id;
             $chbox = html::tag('input', array(
                     'type'    => 'checkbox',
                     'name'    => '_list[]',
                     'value'   => $id,
-                    'checked' => $prop['active'],
+                    'checked' => !empty($prop['active']),
                     'title'   => $this->plugin->gettext('activate'),
                     'aria-labelledby' => $label_id
             ));
@@ -514,16 +514,16 @@ class tasklist_ui
     /**
      *
      */
-    function edit_attendees_notify($attrib = array())
+    function edit_attendees_notify($attrib = [])
     {
-        $checkbox = new html_checkbox(array('name' => '_notify', 'id' => 'edit-attendees-donotify', 'value' => 1, 'class' => 'pretty-checkbox'));
+        $checkbox = new html_checkbox(['name' => '_notify', 'id' => 'edit-attendees-donotify', 'value' => 1, 'class' => 'pretty-checkbox']);
         return html::div($attrib, html::label(null, $checkbox->show(1) . ' ' . $this->plugin->gettext('sendnotifications')));
     }
 
     /**
      * Form for uploading and importing tasks
      */
-    function tasks_import_form($attrib = array())
+    function tasks_import_form($attrib = [])
     {
         if (empty($attrib['id'])) {
             $attrib['id'] = 'rcmImportForm';
@@ -572,33 +572,33 @@ class tasklist_ui
     /**
      * Form to select options for exporting tasks
      */
-    function tasks_export_form($attrib = array())
+    function tasks_export_form($attrib = [])
     {
         if (empty($attrib['id'])) {
             $attrib['id'] = 'rcmTaskExportForm';
         }
 
         $html = html::div('form-section form-group row',
-            html::label(array('for' => 'task-export-list', 'class' => 'col-sm-4 col-form-label'), $this->plugin->gettext('list'))
-            . html::div('col-sm-8', $this->tasklist_select(array(
+            html::label(['for' => 'task-export-list', 'class' => 'col-sm-4 col-form-label'], $this->plugin->gettext('list'))
+            . html::div('col-sm-8', $this->tasklist_select([
                         'name'  => 'source',
                         'id'    => 'task-export-list',
-                        'extra' => array('' => '- ' . $this->plugin->gettext('currentview') . ' -'),
-                )))
+                        'extra' => ['' => '- ' . $this->plugin->gettext('currentview') . ' -'],
+                ]))
         );
 
-        $checkbox = new html_checkbox(array('name' => 'attachments', 'id' => 'task-export-attachments', 'value' => 1, 'class' => 'form-check-input pretty-checkbox'));
+        $checkbox = new html_checkbox(['name' => 'attachments', 'id' => 'task-export-attachments', 'value' => 1, 'class' => 'form-check-input pretty-checkbox']);
         $html .= html::div('form-section row form-check',
-            html::label(array('for' => 'task-export-attachments', 'class' => 'col-sm-4 col-form-label'), $this->plugin->gettext('exportattachments'))
+            html::label(['for' => 'task-export-attachments', 'class' => 'col-sm-4 col-form-label'], $this->plugin->gettext('exportattachments'))
             . html::div('col-sm-8', $checkbox->show(1))
         );
 
         $this->register_gui_object('exportform', $attrib['id']);
 
-        return html::tag('form', array(
-                'action' => $this->rc->url(array('task' => 'tasklist', 'action' => 'export')),
+        return html::tag('form', [
+                'action' => $this->rc->url(['task' => 'tasklist', 'action' => 'export']),
                 'method' => 'post', 'id' => $attrib['id']
-            ),
+            ],
             $html
         );
     }