diff --git a/plugins/kolab_activesync/kolab_activesync.php b/plugins/kolab_activesync/kolab_activesync.php
index 3b5cd96e..2195978b 100644
--- a/plugins/kolab_activesync/kolab_activesync.php
+++ b/plugins/kolab_activesync/kolab_activesync.php
@@ -84,7 +84,7 @@ class kolab_activesync extends rcube_plugin
      */
     function folder_form($args)
     {
-        $mbox_imap = $args['options']['name'];
+        $mbox_imap = $args['options']['name'] ?? '';
 
         // Edited folder name (empty in create-folder mode)
         if (!strlen($mbox_imap)) {
diff --git a/plugins/kolab_folders/kolab_folders.php b/plugins/kolab_folders/kolab_folders.php
index f2d30520..21082d5a 100644
--- a/plugins/kolab_folders/kolab_folders.php
+++ b/plugins/kolab_folders/kolab_folders.php
@@ -219,7 +219,7 @@ class kolab_folders extends rcube_plugin
             return $args;
         }
 
-        if ($args['options']['is_root']) {
+        if (!empty($args['options']['is_root'])) {
             return $args;
         }
 
@@ -231,13 +231,15 @@ class kolab_folders extends rcube_plugin
         }
 
         // Get type of the folder or the parent
+        $subtype = '';
         if (strlen($mbox)) {
             list($ctype, $subtype) = $this->get_folder_type($mbox);
-            if (strlen($args['parent_name']) && $subtype == 'default')
+            if (strlen($args['parent_name']) && $subtype == 'default') {
                 $subtype = ''; // there can be only one
+            }
         }
 
-        if (!$ctype) {
+        if (empty($ctype)) {
             $ctype = 'mail';
         }
 
@@ -297,13 +299,13 @@ class kolab_folders extends rcube_plugin
         $args['form']['props']['fieldsets']['settings']['content']['folderctype'] = array(
             'label' => $this->gettext('folderctype'),
             'value' => html::div('input-group',
-                $type_select->show(isset($new_ctype) ? $new_ctype : $ctype)
-                . $sub_select->show(isset($new_subtype) ? $new_subtype : $subtype)
+                $type_select->show($new_ctype ?? $ctype)
+                . $sub_select->show($new_subtype ?? $subtype)
             ),
         );
 
         $this->rc->output->set_env('kolab_folder_subtypes', $sub_types);
-        $this->rc->output->set_env('kolab_folder_subtype', isset($new_subtype) ? $new_subtype : $subtype);
+        $this->rc->output->set_env('kolab_folder_subtype', $new_subtype ?? $subtype);
 
         $this->add_expire_input($args['form'], $args['name'], $ctype);
 
diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 6977a139..daedcb68 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -169,7 +169,7 @@ class kolab_storage
      * Get a list of storage folders for the given data type
      *
      * @param string Data type to list folders for (contact,distribution-list,event,task,note)
-     * @param boolean Enable to return subscribed folders only (null to use configured subscription mode)
+     * @param bool   Enable to return subscribed folders only (null to use configured subscription mode)
      *
      * @return array List of Kolab_Folder objects (folder names in UTF7-IMAP)
      */
@@ -334,8 +334,9 @@ class kolab_storage
     /**
      * Creates folder ID from folder name
      *
-     * @param string  $folder Folder name (UTF7-IMAP)
-     * @param boolean $enc    Use lossless encoding
+     * @param string $folder Folder name (UTF7-IMAP)
+     * @param bool   $enc    Use lossless encoding
+     *
      * @return string Folder ID string
      */
     public static function folder_id($folder, $enc = null)
@@ -850,7 +851,7 @@ class kolab_storage
      * @param string  Optional root folder
      * @param string  Optional name pattern
      * @param string  Data type to list folders for (contact,event,task,journal,file,note,mail,configuration)
-     * @param boolean Enable to return subscribed folders only (null to use configured subscription mode)
+     * @param bool    Enable to return subscribed folders only (null to use configured subscription mode)
      * @param array   Will be filled with folder-types data
      *
      * @return array List of folders
@@ -1216,18 +1217,20 @@ class kolab_storage
      * @param string $folder Folder name
      * @param string $type   Content type
      *
-     * @return boolean True on success
+     * @return bool True on success
      */
-    public static function set_folder_type($folder, $type='mail')
+    public static function set_folder_type($folder, $type = 'mail')
     {
         self::setup();
 
-        list($ctype, $subtype) = explode('.', $type);
+        list($ctype, $subtype) = strpos($type, '.') !== false ? explode('.', $type) : array($type, null);
 
         $success = self::$imap->set_metadata($folder, array(self::CTYPE_KEY => $ctype, self::CTYPE_KEY_PRIVATE => $subtype ? $type : null));
 
-        if (!$success)  // fallback: only set private annotation
+        if (!$success) {
+            // fallback: only set private annotation
             $success |= self::$imap->set_metadata($folder, array(self::CTYPE_KEY_PRIVATE => $type));
+        }
 
         return $success;
     }
@@ -1236,9 +1239,9 @@ class kolab_storage
      * Check subscription status of this folder
      *
      * @param string $folder Folder name
-     * @param boolean $temp  Include temporary/session subscriptions
+     * @param bool   $temp  Include temporary/session subscriptions
      *
-     * @return boolean True if subscribed, false if not
+     * @return bool True if subscribed, false if not
      */
     public static function folder_is_subscribed($folder, $temp = false)
     {
@@ -1257,9 +1260,9 @@ class kolab_storage
      * Change subscription status of this folder
      *
      * @param string $folder Folder name
-     * @param boolean $temp  Only subscribe temporarily for the current session
+     * @param bool   $temp   Only subscribe temporarily for the current session
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public static function folder_subscribe($folder, $temp = false)
     {
@@ -1287,9 +1290,9 @@ class kolab_storage
      * Change subscription status of this folder
      *
      * @param string $folder Folder name
-     * @param boolean $temp  Only remove temporary subscription
+     * @param bool   $temp   Only remove temporary subscription
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public static function folder_unsubscribe($folder, $temp = false)
     {
@@ -1315,7 +1318,7 @@ class kolab_storage
      *
      * @param string $folder Folder name
      *
-     * @return boolean True if active, false if not
+     * @return bool True if active, false if not
      */
     public static function folder_is_active($folder)
     {
@@ -1329,7 +1332,7 @@ class kolab_storage
      *
      * @param string $folder Folder name
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public static function folder_activate($folder)
     {
@@ -1343,7 +1346,7 @@ class kolab_storage
      *
      * @param string $folder Folder name
      *
-     * @return True on success, false on error
+     * @return bool True on success, false on error
      */
     public static function folder_deactivate($folder)
     {
@@ -1591,8 +1594,8 @@ class kolab_storage
     /**
      * Get a list of (virtual) top-level folders from the other users namespace
      *
-     * @param string  Data type to list folders for (contact,event,task,journal,file,note,mail,configuration)
-     * @param boolean Enable to return subscribed folders only (null to use configured subscription mode)
+     * @param string Data type to list folders for (contact,event,task,journal,file,note,mail,configuration)
+     * @param bool   Enable to return subscribed folders only (null to use configured subscription mode)
      *
      * @return array List of kolab_storage_folder_user objects
      */