There actually is some nice tcp/ip based server called userserver, which you can have running on a Windows machine in your domain that will serve information about users on request.
Besides the .exe the package comes with an example .php script which let you communicate with the userserver through simple tcp/ip stuff (you simply define the location of the userserver service 😉
In the userserver you will see methods like is_In_Group and is_valid_user ($login, $password) (as far as I remember)..
You should be able to find it on the net.. otherwise lemme know and I'll hook you up with it.
Martin
