if (!empty($_SERVER['HTTP_REFERER'])) {
        $url = parse_url($_SERVER['HTTP_REFERER']);
        $host = $url['host'];
        $ip = gethostbyname($host);
        if ($ip != $host) {
            if ($host != $_SERVER['SERVER_NAME']) {
                die("STOP ! Illegal action from '$host' (IP=$ip)");
            }
        }
    }