private function readTicket()
    {
        if (!$this->getFileSystem()->isFile($this->getTicketFilePath())) {
            return '';
        }

        $content = trim($this->getFileSystem()->get($this->getTicketFilePath()));
        $result = unserialize($content);

        if (isset($result['ticket']) && isset($result['expire']) && isset($result['time'])) {
            $expireTime = $result['time'] + $result['expire'] - self::EXPIRE_SUB_TIME;
            if ($expireTime > time()) {
                return $result['ticket'];
            }
        }

        return '';
    }