As a part of his work on his entry into the WinPHP challenge, Juozas Kaziukenas has posted a method he's figured out to get PHP and Silverlight talking.
Only on rare cases applications created with Silverlight (or Flash) are static - it’s very common to have information coming from a RSS feed, REST service or any other data source. What is more important, these applications usually needs configuration variables (user name, language, products category, etc.) to be passed to them. But how to do all that?
He shows how to create a simple configuration that can be read by Silverlight and used to pass information back into your PHP script. There's even a more asynchronous method that lets your messaging work a lot like Ajax.