include external php apps

[php-phan]

Hello. So i am trying to create something somewhat like what facebook did with their platform, just as like a little learning experience thingy. So this was how everything was going to go down.

www.mysite.com?app=Bejeweled

then i have loader.php and that $_GET['app'] and finds the application in the database that has the name "Bejeweled" it then gets the URL were the application is at and includes(); it.
My problem is, in the loader.php i have to connect to the database, to do that I created a CRUD class where i can just call $crud->select('table', '*', ''name'='Bejeweled'); but after this i do include(APPLICATION_URL); but I dont want the app to have access to the $crud class or any other server functions....

Does anyone have a suggestion as to how this could be accomplished SECURELY. This is the only way i can think of doing it. Also I dont wanna waste my servers space so the application will have to be hosted on another site...

Thanks in advanced :-)

[Kudose]

I am pretty sure Facebook uses iframes.

The only way I can think of to securely execute remote PHP code is for the output of the included external site to be PHP code and to have some sort of token/checksum system to verify the content.

Unless you control the external site, I would be very weary about doing this as they could easily compromise your server.