diff --git a/lib/api/file_create.php b/lib/api/file_create.php
--- a/lib/api/file_create.php
+++ b/lib/api/file_create.php
@@ -41,10 +41,11 @@
             }
         }
 
+        $is_file = false;
         if (is_resource($this->args['content'])) {
             $chunk = stream_get_contents($this->args['content'], 1024000, 0);
         }
-        else if ($this->args['path']) {
+        else if ($this->args['path'] ?? null) {
             $chunk   = $this->args['path'];
             $is_file = true;
         }
@@ -60,8 +61,8 @@
         $request = $this instanceof file_api_file_update ? 'file_update' : 'file_create';
         $file    = array(
             'content' => $this->args['content'],
-            'path'    => $this->args['path'],
-            'type'    => rcube_mime::file_content_type($chunk, $this->args['file'], $ctype, !$is_file),
+            'path'    => $this->args['path'] ?? null,
+            'type'    => rcube_mime::file_content_type($chunk, $this->args['file'] ?? null, $ctype, !$is_file),
         );
 
         if (strpos($file['type'], 'empty') !== false && $ctype) {
@@ -80,7 +81,7 @@
 
         $driver->$request($path, $file);
 
-        if (rcube_utils::get_boolean((string) $this->args['info'])) {
+        if (rcube_utils::get_boolean((string) ($this->args['info'] ?? null))) {
             return $driver->file_info($path);
         }
     }
diff --git a/lib/api/file_info.php b/lib/api/file_info.php
--- a/lib/api/file_info.php
+++ b/lib/api/file_info.php
@@ -34,8 +34,8 @@
         // check Manticore support. Note: we don't use config->get('fileapi_manticore')
         // here as it may be not properly set if backend driver wasn't initialized yet
         $capabilities = $this->api->capabilities(false);
-        $manticore    = $capabilities['MANTICORE'];
-        $wopi         = $capabilities['WOPI'];
+        $manticore    = $capabilities['MANTICORE'] ?? null;
+        $wopi         = $capabilities['WOPI'] ?? null;
 
         // support file_info by session ID
         if (!isset($this->args['file']) || $this->args['file'] === '') {
diff --git a/lib/api/file_list.php b/lib/api/file_list.php
--- a/lib/api/file_list.php
+++ b/lib/api/file_list.php
@@ -36,7 +36,7 @@
         }
 
         $params = array(
-            'reverse' => rcube_utils::get_boolean((string) $this->args['reverse']),
+            'reverse' => rcube_utils::get_boolean((string) ($this->args['reverse'] ?? "")),
         );
 
         if (!empty($this->args['sort'])) {
diff --git a/lib/api/quota.php b/lib/api/quota.php
--- a/lib/api/quota.php
+++ b/lib/api/quota.php
@@ -31,14 +31,14 @@
     {
         parent::handle();
 
-        list($driver, $path) = $this->api->get_driver($this->args['folder']);
+        list($driver, $path) = $this->api->get_driver($this->args['folder'] ?? null);
 
         $quota = $driver->quota($path);
 
-        if (!$quota['total']) {
+        if (!($quota['total'] ?? false)) {
             $quota['percent'] = 0;
         }
-        else if ($quota['total']) {
+        else if ($quota['total'] ?? false) {
             if (!isset($quota['percent'])) {
                 $quota['percent'] = min(100, round(($quota['used']/max(1, $quota['total']))*100));
             }
diff --git a/lib/drivers/kolab/kolab_file_storage.php b/lib/drivers/kolab/kolab_file_storage.php
--- a/lib/drivers/kolab/kolab_file_storage.php
+++ b/lib/drivers/kolab/kolab_file_storage.php
@@ -179,6 +179,7 @@
 
         // parse $host
         $a_host = parse_url($host);
+        $port = null;
         if ($a_host['host']) {
             $host = $a_host['host'];
             $ssl = (isset($a_host['scheme']) && in_array($a_host['scheme'], array('ssl','imaps','tls'))) ? $a_host['scheme'] : null;
@@ -268,7 +269,7 @@
     {
         $this->rc->plugins->exec_hook('startup');
 
-        if ($_SESSION['user_id'] || $user) {
+        if ($_SESSION['user_id'] ?? false || $user) {
             // overwrite config with user preferences
             $this->rc->user = $user ? $user : new rcube_user($_SESSION['user_id']);
             $this->rc->config->set_user_prefs((array)$this->rc->user->get_prefs());
@@ -652,7 +653,7 @@
 
         return array(
             'name'     => $file['name'],
-            'size'     => (int) $file['size'],
+            'size'     => (int) ($file['size'] ?? null),
             'type'     => (string) $file['type'],
             'mtime'    => file_utils::date_format($file['changed'], $this->config['date_format'], $this->config['timezone']),
             'ctime'    => file_utils::date_format($file['created'], $this->config['date_format'], $this->config['timezone']),
@@ -702,11 +703,11 @@
                 continue;
             }
 
-            $filename = $params['prefix'] . $folder_name . file_storage::SEPARATOR . $file['name'];
+            $filename = ($params['prefix'] ?? null) . $folder_name . file_storage::SEPARATOR . ($file['name'] ?? null);
 
             $result[$filename] = array(
                 'name'     => $file['name'],
-                'size'     => (int) $file['size'],
+                'size'     => (int) ($file['size'] ?? null),
                 'type'     => (string) $file['type'],
                 'mtime'    => file_utils::date_format($file['changed'], $this->config['date_format'], $this->config['timezone']),
                 'ctime'    => file_utils::date_format($file['created'], $this->config['date_format'], $this->config['timezone']),
@@ -1003,7 +1004,7 @@
         // This could probably be optimized by doing a direct
         // IMAP LIST command with prepared second argument, but
         // it would make caching not optimal
-        if ($params['level'] > 0) {
+        if (($params['level'] ?? 0) > 0) {
             $offset = isset($params['path']) && strlen($params['path']) ? strlen($params['path']) + 1 : 0;
             foreach ($folders as $idx => $folder) {
                 if (substr_count($folder, $separator, $offset) >= $params['level']) {
@@ -1592,7 +1593,7 @@
      */
     protected function from_file_object($file)
     {
-        if (isset($file['filename']) && !$file['name']) {
+        if (isset($file['filename']) && !($file['name'] ?? false)) {
             $file['name'] = $file['filename'];
         }
 
@@ -1626,7 +1627,7 @@
                 'path'     => $file['path'],
                 'content'  => $file['content'],
                 'mimetype' => $file['type'],
-                'size'     => $file['size'],
+                'size'     => $file['size'] ?? null,
         ));
 
         unset($file['name']);
diff --git a/lib/file_api.php b/lib/file_api.php
--- a/lib/file_api.php
+++ b/lib/file_api.php
@@ -60,7 +60,7 @@
         $this->request = strtolower($_GET['method']);
 
         // Check the session, authenticate the user
-        if (!$this->session_validate($this->request == 'authenticate', $_REQUEST['token'])) {
+        if (!$this->session_validate($this->request == 'authenticate', $_REQUEST['token'] ?? null)) {
             $this->session->destroy(session_id());
             $this->session->regenerate_id(false);
 
@@ -73,7 +73,7 @@
                 $_SESSION['env']  = $this->env;
 
                 // remember client API version
-                if (is_numeric($_GET['version'])) {
+                if (is_numeric($_GET['version'] ?? null)) {
                     $_SESSION['version'] = $_GET['version'];
                 }
 
@@ -124,7 +124,7 @@
 
         // Single-document session?
         if (!($this instanceof file_api_wopi)
-            && ($doc_id = $_SESSION['document_session'])
+            && ($doc_id = ($_SESSION['document_session'] ?? null))
             && (strpos($this->request, 'document') !== 0 || $doc_id != $_GET['id'])
         ) {
             throw new Exception("Access denied", file_api_core::ERROR_UNAUTHORIZED);
@@ -316,7 +316,7 @@
                 $request = 'document';
             }
 
-            $request = $aliases[$request] ?: $request;
+            $request = $aliases[$request] ?? $request;
 
             require_once __DIR__ . "/api/common.php";
             include_once __DIR__ . "/api/$request.php";
diff --git a/lib/file_api_core.php b/lib/file_api_core.php
--- a/lib/file_api_core.php
+++ b/lib/file_api_core.php
@@ -279,12 +279,12 @@
             return $caps;
         }
 
-        if ($caps['MANTICORE']) {
+        if ($caps['MANTICORE'] ?? false) {
             $manticore = new file_manticore($this);
             $caps['MANTICORE_EDITABLE'] = $manticore->supported_filetypes(true);
         }
 
-        if ($caps['WOPI']) {
+        if ($caps['WOPI'] ?? false) {
             $wopi = new file_wopi($this);
             $caps['WOPI_EDITABLE'] = $wopi->supported_filetypes(true);
         }