How can I get invited user's id?

I want to add a function on my facebook application that enable to chec开发者_JS百科k user ids which is invited by certain user. e.g. A invited B, then insert id of B relating with id of A to Database only for the first request. (It doesn't matter whether B will accept the request or not) I will add up how many people was invited by A and give some items according to the count on my game application. Something like that.

I'm not sure how can I make it.

I could get solution by myself.
$request_url ="https://graph.facebook.com/REQUEST_ID?ACCESS_TOKEN; $requests = file_get_contents($request_url);