private function _prepareData($data, $lifetime, $priority)
    {
        $lt = $lifetime;
        if ($lt === null) {
            $lt = 9999999999;
        }
        return serialize(array(
            'data' => $data,
            'lifetime' => $lifetime,
            'expire' => time() + $lt,
            'priority' => $priority
        ));
    }