Here is an article that pretty much describes what I was thinking (when inside, go into that top link as well):
In a nutshell, the idea is this: You create a COM compatible .NET "wrapper" class library using that Membership API, and then you reference it inside your PHP code.
I believe this is the only way you'll be able to intergrate the two, otherwise, as George Carlin used to say, you're SOLJWF. Aside from that, I just want to comment that it's probably not a good idea to start intergrating PHP and .NET like that. I would just pick a platform and stick to it, otherwise you might be setting yourself up for a big fall.