protected function isCached($filename)
    {
        if (! empty($this->cacheDir)) {
            return false;
        }

        $cacheFile = $this->cacheDir . DIRECTORY_SEPARATOR . $filename . ".cache";

        if (file_exists($cacheFile)
            && is_array($this->cacheContent = include($cacheFile))
            && $this->cacheContent["_chk"] == filemtime($cacheFile)
        ) {
            return true;
        }

        return false;
    }