How to get the sending user's id from the callback page with fb-request-form?

im sending invitation requests via the request-form to my application and i want to the sending users id from the action page. i can successfully get the ids the invitations are sent to but am not sure how to get the senders id.

im using php. this is part of my callback page:

 if (isset($_REQUEST['ids'])){
    $amount = sizeof($_REQUEST['ids']);
    print_r($_REQUEST);
            echo "<br>$amount invitations Successfully Sent";
}

the request prints out like this:

Array ( [mfs_typeahead_req_form_4cf74f96db3688507476560] => Start Typing a Name [ids] => Array ( [0] => 510149460 ) [fbs_258181898823] => \"access_token=258181898823|2.q_qb_yoReO0_xc4H8PxKRQ__.3600.1291280400-开发者_Python百科100000664203700|LqtGr_OiJTASGmek61awxmxfvFk&expires=1291280400&secret=85eTEELZj8lkV82V_PwRSA__&session_key=2.q_qb_yoReO0_xc4H8PxKRQ__.3600.1291280400-100000664203700&sig=d4cc0e4c0992ea29c0adfd60dc27185b&uid=100000664203700\" )

the id i want is 100000664203700

If using requets 2.0 http://developers.facebook.com/docs/reference/dialogs/requests/

You can get the user id form current session and append to the data parameter in the request. Facebook will send the data though as a string and you can extract on the redirect uri.

as per your print_r() Have you tried json decoding the $_REQUEST? This will give you an array to work with and you can do a quick search for uid= in your string.

http://php.net/manual/en/function.json-decode.php