facebook uninstall application callback

My problem is that I want to know the users that uninstall my facebook application. I have set a callback url for facebook to hit when someone uninstalls the app and all I get is a signed request POST variable which, according to facebook documentation should be a base64 json encoded object but it seems impossible to trace the user's facebook id in there. Actually it's more like random data.. :| Anybody knows how to parse this thing to get the fb uid?

I tried:

<?php
$data = json_decode(base开发者_如何转开发64_decode($_POST['signed_request']));
?>

And still I don't get something valuable out of it..

Check this documentation on facebook: http://developers.facebook.com/docs/authentication/signed_request/ Especially the "Verifying and Decoding" section, where you will find code to decode your signed_request parameter.