private function _generateUniqueId()
    {
        $rand = uniqid(rand(10, 99));
        $time = microtime(true);
        $micro = sprintf('%06d', ($time - floor($time)) * 1000000);
        $date = new DateTime(date('Y-m-d H:i:s.' . $micro, $time));
        return sha1($date->format('YmdHisu'));
    }