Facebook Application Requests: How to determine who's invited whom?

We're about to build a Facebook application that will run inside an XFBML IFrame. We already found a good tutorial for that (kudos to the author). Right now, there's one more issue we need to deal with.

开发者_运维技巧Basically, we just wanted to know how to determine the UID of the person who invited a friend to join an application. Say for example, user A invites user B to join Cool-App. User B sees the application request, and responded by accepting the invitation. Now the question is, how are we going to know that user B was referred by user A?

Help please.

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Well, luckily we found the closest possible answer.

FBML has fb:req-choice tag which we normally use inside fb:request-form. Can also be embedded in the Content attribute of the latter.

It looks like this:

<fb:req-choice url="URL_HERE" label="BUTTON_TEXT_HERE"/>

..where URL_HERE is your application-base-url plus a relative path to somewhere like so:

http://apps.facebook.com/my_cool_app/path/to/somewhere

e.g:

http://apps.facebook.com/my_cool_app/users/invite/referred-by/1234567890

This actually is the button which the user "accepts" or "confirms". When triggered, FB redirects to the link you provided.

That's it. The rest is up to your application.

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When using the FBML request-form tag, you can retrieve the invited users as a POST var.
php example:

if(isset($_POST["ids"])) {
    echo "<center>Thank you for inviting ".sizeof($_POST["ids"])." of your friends on <b><a href=\"http://apps.facebook.com/".$app_url."/\">".$app_name."</a></b>.<br><br>\n";
    echo "<h2><a href=\"http://apps.facebook.com/".$app_url."/\">Click here to return to ".$app_name."</a>.</h2></center>";
}

There is no way that I know of to find out if the invited user actually accepted this particular invitation. (f.e. What if he also gets a request from user C?)