Using windows authentication with php?
Try LDAP
I've written intranet web applications that use the local windows username and password to authenticate and LDAP is by far the best solution.
If you are using LDAP at work, go with that, if not, well you have to access it thru .NET.
Since there are several but not fully reliable .NET implementations for PHP, I would recommend using a WebService, which PHP would consume, that will pass the user and pass - encrypted would be nice - to it, and it will return if the credentials are valid or not.
You should be able to use LDAP, in your case it is most likely Windows Active Directory via IIS. There are php packages out there to interact with various types of LDAP implementations. Here is a link:
http://php.net/manual/en/book.ldap.php