public static function _sendCacheHeaders($expire = 5) {
        if (!is_numeric($expire) || $expire < 1 || $expire > 240) {
            $expire = 5;
            _Log::warn("Attempt to send cache headers with invalid expiration");
        }
        header("Cache-Control: public"); // HTTP/1.1
        header("Expires: " . gmdate("D, d M Y H:i:s", time() + 60 * $expire) . " GMT");
        header("Last-Modified: " . gmdate('D, d M Y H:i:s') . " GMT");
    }