If you're using IIS and have a domain controller you can go into the IIS Management Console and disallow annonymous logons. This will prompt a user for their domain username/password. If login is successful, you can obtain the username with $_SERVER['PHP_AUTH_USER'].
If you're using Apache, you can probably do something similar using a .htaccess file, but I'm less familar with that.
If you're looking for a way to determine what credentials were used to log into Windows, and not have any authentication on your site, PHP is not going to be able to do it. Perhaps something that's client side could though.