facebook朋友邀请 - 谁发送了邀请

Question

I have an app where each user can invite his friends to join the app.

I have done this via the JS FB SDK.

I also use PHP SDK for server sessions and other things.

Now the questions is how can I determine the id of the inviter when an user joins my app based on the invitation sent?

EG: Let's say USER-X invites USER-Z to join my app. Then USER-Y invites USER-Z to join my app too. But USER-Z joins through the link of USER-X. I want to reward USER-X because he brought a new user to my app, so I need his fbid or something. How can I do determine the referer?

I have used this code to implement the Facebook friends invite on my website:

<script src="https://connect.facebook.net/en_US/all.js"></script>
<script type="text/javascript">
 FB.init({
  appId:'<?=APPID?>',
  cookie:true,
  status:true,
  xfbml:true
 });
  function FBInvite(){
  FB.ui({
   method: 'apprequests',
   message: 'Invite your Facebook Friends'
  },function(response) {
   if (response) {
    alert('Successfully Invited');
   } else {
    alert('Failed To Invite');
   }
  });
 }
</script>

<a href="#" onclick="FBInvite();">OPEN INVITE POPUP</a>