public static function cacheHeaders($expires = '+1 hour')
    {
        if (!headers_sent()) {
            header('Cache-Control: private, max-age=3600');
            header('Pragma: public');
            header('Last-Modified: ' . date(DATE_RFC822, strtotime('-1 second')));
            header('Expires: ' . date(DATE_RFC822, strtotime($expires)));
        }
    }