Matthias Shapiro has published this three-part tutorial in which is explained how to create a very simple web service in PHP that pulls from a MySQL database and to display the data in Silverlight. Here you can read these posts:
- PHP, MySQL, and Silverlight: The Complete Tutorial (Part 1)
- PHP, MySQL, and Silverlight: The Complete Tutorial (Part 2)
- PHP, MySQL, and Silverlight: The Complete Tutorial (Part 3)