private function filename($what = null) {
        // no files?
        if(!$this->$what['intern'])
            return false;

        $last = 0;
        foreach($this->$what['intern'] as $res)
            if(file_exists($res))
                $last = max($last, filemtime($res));

        return "cache-{$last}-" . md5(serialize($this->$what['intern'])) . ".{$what}";
    }