public function getTtl($key)
    {
        $getResult = $this->getValue($key);
        $unserialized = @unserialize($getResult);

        if (!Util::hasInternalExpireTime($unserialized)) {
            throw new \Exception('Cannot retrieve ttl');
        }

        return $this->handleTtl($key, $unserialized['ts'], $unserialized['s']);
    }