<?php 
 
     $app_id = "[YOUR APP ID]";
 
     $canvas_page = "[YOUR APP URL]";
 
     $auth_url = "http://www.facebook.com/dialog/oauth?client_id=" 
            . $app_id . "&redirect_uri=" . urlencode($canvas_page);
 
     $signed_request = $_REQUEST["signed_request"];
 
     list($encoded_sig, $payload) = explode('.', $signed_request, 2); 
 
     $data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);
 
     if (empty($data["user_id"])) {
            echo("<script> top.location.href='" . $auth_url . "'</script>");
     } else {
            echo ("Welcome User: " . $data["user_id"]);
     } 
 ?>