What should I use for session_id for a facebook IFrame app?

I'm working on an IFrame开发者_运维知识库 apps in Facebook.

I use the PHP SDK provided in facebook. and I can login. However, what should I use for session id in PHP? should I use the default PHP session id (by the server)? or should I use the 'sig' that I got from the facebook SDK?

(i.e. the $session = $facebook->getSession() , $session['sig'] )

I'm confused by the getSession() from that PHP SDK, and what should be set i my server.

If you want to do facebook related operation then you have to have $session id returned from facebook. You cannot retrieve user information from facebook with your session id of server. $session_id returned from facebook, means facebook authenticates that user through application. Then all the information will be provided on the basis of that $session id.