public function set($key, $data, $ttl = null)
    {
        if ($ttl) {
            $now = new \DateTime('now', new \DateTimeZone('UTC'));
            $expire = (int) $now->format('U') + $ttl;
        } else {
            $expire = 0;
        }

        $entry = array(
            'expire' => $expire,
            'value' => $data
        );

        $this->storage[$key] = $entry;

        return true;
    }