How to find out a user is logout or not in Facebook iFrame App?

My application is iFrame App. Before, I can use $_REQUEST['fb_sig_logged_out_facebook'] to determine the facebook user is logout or not. However, with iFrame, I don't see that variable passing in. What can I do?

I'm also using the new PHP SDK but it doesn't have any function to help me on that.

What I want to find out is whether the user has logged out facebook or not installed the application. Both will return null in the facebook->session. I 开发者_开发技巧need to distinguish these two. :)

Facebook no longer provides that querystring value with the new authentication system. Facebook's migration guide states that you can "Assumed when there is not a profile_id and your application is an IFrame app." You can read more about the new canvas authentication here: http://developers.facebook.com/docs/authentication/canvas

The other alternative is to check if the window.top.location is on the apps.facebook.com domain.