Can you get a Windows (AD) username in PHP?

Check the AUTH_USER request variable. This will be empty if your web app allows anonymous access, but if your server's using basic or Windows integrated authentication, it will contain the username of the authenticated user.

In an Active Directory domain, if your clients are running Internet Explorer and your web server/filesystem permissions are configured properly, IE will silently submit their domain credentials to your server and AUTH_USER will be MYDOMAIN\user.name without the users having to explicitly log in to your web app.

I've got php mysql running on IIS - I can use $_SERVER["AUTH_USER"] if I turn on Windows Authentication in IIS -> Authentication and turn off Anonymous authentication (important)

I've used this to get my user and domain:

$user = $_SERVER['AUTH_USER'];

$user will return a value like: DOMAIN\username on our network, and then it's just a case of removing the DOMAIN\ from the string.

This has worked in IE, FF, Chrome, Safari (tested) so far.

Look at the PHP LDAP library functions: http://us.php.net/ldap.

Active Directory [mostly] conforms to the LDAP standard.