On the "Make Me Pulse" blog today Ludovic Hindryckx has a bit of code he's shared to let you connect PHP to an Active Directory server (via an LDAP connection).
Today we are going to see how to make an authentication with PHP and an Active Directory LDAP (AD). [...] This code sample has been tested on a Windows 2k3 server.
The script calls the ldap_connect connect function with the user-submitted username and password and runs a bind (as the user) and searches for an account with the same name. If the information is found, it returns back to the $info variable to be checked.