public function get_cache($name, $type='db', $ttl=0, $packed=true)
    {
        if (!isset($this->caches[$name]) && ($userid = $this->get_user_id())) {
            $this->caches[$name] = new rcube_cache($type, $userid, $name, $ttl, $packed);
        }

        return $this->caches[$name];
    }