PHP NTLM：凭证失败时阻止登录表单

Question

I'm using the normal (I think is standard) PHP NTLM:

$headers = apache_request_headers();
if (!isset($headers['Authorization'])) {
    header('HTTP/1.1 401 Unauthorized');
    header('WWW-Authenticate: NTLM');
    exit;
}

$auth = $headers['Authorization'];
if (substr($auth,0,5) == 'NTLM ') {
    ....
}

I notice that if the user had login into his windows domain, my script will be revoked 3 times (NTLM credential detected). But if NTLM failed, the browser will show login form.

Now I don't want this login form. I need to detect if NTLM credential failed, instead of showing login form, I want to redirect to other page (eg Page Sorry You Are Not Domain User).

If I change my code to this, even user with valid NTLM credential will be redirected:

$headers = apache_request_headers();
if (!isset($headers['Authorization'])) {
    redirect to other page
}

$auth = $headers['Authorization'];
if (substr($auth,0,5) == 'NTLM ') {
    ....
}