public function isCached()
    {
        $cache_lifetime = Helper::getOption('cache_lifetime', 0);
        if ($cache_lifetime>0) {
            return (@file_exists($this->getCacheKey()) && (filemtime($this->getCacheKey())+$cache_lifetime) > time());
        } else {
            return (@file_exists($this->getCacheKey()));
        }
    }