This post is obsolete. Read New post How to run PHP in Webmatrix (updated) ? regarding same topic.
Do you have tried running PHP application in webmatrix. You can run them as well your ASP.NET application.
Here is two solution for you if you are unable to run your own PHP application on your computer.
1. try to check C:\Program Files (x86)\IIS Express\PHP/V if you are unable to find this directory then it's means you don't have install PHP for webmatrix. You need to install them through Web plateform installer.
2. If first option is OK for you and you are still not able to run your own application then check that if you are enable PHP or not for your website.
for checking go to Site > Settings > PHP settings. Check the checkbox and it's will run your php application.
Brian's post on creating PHP websites in webmatrix
RuslanY's post on How to enable Xdebug in WebMatrix