I've done something like this and there's a simple method I worked on early in my career. You'll need to know how AD works in order to properly authenticate with it.
The theory behind it is using the AD connection functions provided in PHP. Basically you'll use php to connect to AD and bind to it. If the binding succeeds, then the user account is valid. If the bind fails, then there's a problem with the login information. I'm not 100% sure what happens with user accounts that are disabled, but it's something to test for.
You can use a class like this...
http://adldap.sourceforge.net/
Or build a custom solution like this guy...
http://bytes.com/forum/thread633375.html
This function is pretty much what your looking for...
http://www.php.net/ldap_bind
The hardest part for me was learning how LDAP worked, but once you figure it out, it's pretty easy to work with.