Authenticate user serverside from stand alone application

We have a separate app, a completely stand alone site. We use faceboo开发者_开发百科k for simplified login.

We do the following right now

1. show fb login
2. authenticate user with fb javascript
3. create our own account in our db and link it with their fb id.

all that works great. But now the user goes to a new computer, he first visits fb and logs in. He then goes to our site. Since we are pre-approved before we want to automatically log the user in with our own account.

So, simple question, how can i serverside (not javascript) check if the user is authenticated logged in with fb and also approved our app. I have the users fbID

Common way to check user authentication is to use Javascript SDK, its FB.init function. This function makes request to http://www.facebook.com/extern/login_status.php which return authentication info. I think, you can do this request serverside.