If you are looking to do an HTTP request at a lower level then HttpWebRequest is the class that you're looking for. Here, we take a look at doing an HTTP POST from a Silverlight application using this class.
that should work. íts just a regular webrequest, nothing .net specific you'd get the html from the php page in your response strem and just read that and xml and do whatever to it. the same would be true for posting
you can host silverlight on any backend (including lamp) you only have to add the mime types
Silverlight makes very little requirement as to what kind of web server it's served off ( you probably need to add the MIME type for the XAP extension ) so it can be IIS or A.N.Other web server.
Equally, it makes very little requirement about what kind of web servers you talk to whether that's via web services, WebClient, HttpWebRequest or whatever.