protected function read($file)
    {
        $data = null;
        $key  = $file;
        $file = $this->getFileName($file);

        if ( file_exists($file))
        {
            // if cache file not expire
            if (filemtime($file) >= time())
            {
                echo "read cache<br/>";
                $data = file_get_contents($file);
                $data = ($this->base64Encode) ? base64_decode($data) : $data;
                $data = unserialize($data);
                static::$keys[$key] = true;
            }
            else
            {
                unset(static::$keys[$key]);
                unlink($file);
            }
        }

        return $data;
    }