Hello, I don't know PHP, and I can't run your web site on an Apache server. It gives me a lot of script error... But with Expression Web 2, I got an overview of your homepage index.php. It looks like you have an application called Shout box. It looks like a program that allows users to post messages. You have a series of pictures to represent the various expressions. It looks like a forum.
If this is what you want to redesign with Silverlight, first I must warn you currently Silverlight doesn't have well support of documents and rich text input. You may have to overlay html rich text box on top of the Silverlight content.
Also I've noticed you've embedded too many SQL queries and dynamic constructed html in the PHP pages. I don't know if this is a recommended approach in PHP, but I don't think it's a good option. Silverlight works best with SOA (service oriented architecture), where you have some web services (either written by yourself or third party) that get data from database, xml files, or any other media. Then your Silverlight application will call the web services to get the data, and present them to the user. You can also get user inputs in the Silverlight application, and call web services to store the new data on the server. Silverlight also recommends you to separate markup from code, so a designer can work on the markup while a developer can work on the code. This approach also have a lot of benefits on site maintenance. By your current approach, I'm afraid you'll have to completely redesign your architecture...
So if you want to use Silverlight to enhance your web site, I think you can create some small Silverlight contents, such as a navigation bar, and host the Silverlight contents in your PHP pages. Or if you have ambitions, you can completely recreate your site to use an SOA architecture and AJAX client programming model. But even though, you should know Silverlight has limitations. It's very difficult to create a whole web site in Silverlight. You can try to convert some modules that don't require rich text to Silverlight, such as an account management system, where the users can modify their profiles, manage friends, view the links to their recent posts, etc.
shanaolanxing - Please mark the posts as answers if they help and unmark if they don't.