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!