This link talks quite a lot about .NET and PHP5 interconnecting, though I don't know precisely what it says, because the site is in French. In summary, though, I think it hits these points:
- For tight coupling, PHP can do COM interop. This was solid in PHP 4. While the XML support in PHP was still maturing, I was writing PHP apps that used MSXML4 to do fast parsing and XSLT. Simple and easy.
- PHP5 includes new DOTNET support, so you can create an instance of any public .NET class. They give an example of a PHP app instantiating a System.Collections.Stack. Simple. You can also do this with your own custom classes and assemblies. This is another form of tight coupling.
- PHP and .NET can interconnect via loosely-coupled techniques. Swapping XML streams back and forth is simple and easy. Either .NET or PHP can be either the producer or consumer of the XML.
- Another variation on that idea is SOAP. NuSOAP is apparently maturing. Last I tried it, 1+ year ago, it didn't work very well!
- There are some more exotic options, like, PHP#, which is a variant of PHP that runs on the CLR, and a PHP add-in tool for VS.NET
Worth checking out!