$facebook->getSignedRequest(); return the correct value in iframe while return null on my server side - api

Question

i am building a fan gate for my site to give out some coupon.
here is my code:
<?php
require_once 'facebook.php';
$app_id = "xxxxxxxxxxxxxxx";
$app_secret = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
$facebook = new Facebook(array(
'appId' => $app_id,
'secret' => $app_secret,
'cookie' => true
));
print_r($_REQUEST);
$signed_request = $facebook->getSignedRequest();
echo "<pre>";
print_r($facebook->getSignedRequest());
echo "</pre>";
$signed_request = $_REQUEST["signed_request"];
list($encoded_sig, $payload) = explode('.', $signed_request, 2);
$data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);
$like_status = $signed_request["page"]["liked"];
print_r($facebook);
?>
I find that the return value of $facebook->getSignedRequest(); is different when i am visiting my apps via apps.facebook.com/xxxxxxxxxxxxxxxxxxxxxx/ and when i am visiting www.coupon.mysite.com/facebook/index.php
via apps.facebook.com/xxxxxxxxxxxxxxxxxxxxxx/:
Facebook Object ( [appId:protected] => 15255288xxxxxxxx [appSecret:protected] => XXXXXXXXXXXXXXXXXXXX[user:protected] => [signedRequest:protected] => Array ( [algorithm] => HMAC-SHA256 [issued_at] => xxxxxxxxxxxxx[user] => Array ( [country] => hk [locale] => en_US [age] => Array ( [min] => 21 ) ) ) [state:protected] => [accessToken:protected] => [fileUploadSupport:protected] => )
via www.coupon.mysite.com/facebook/index.php:
Facebook Object ( [appId:protected] => xxxxxxxxxxxxxxxxxxx[appSecret:protected] => xxxxxxxxxxxxxxxxxx[user:protected] => [signedRequest:protected] => [state:protected] => [accessToken:protected] => [fileUploadSupport:protected] => )
so the fan gate wont work because $facebook->getSignedRequest(); always return null in my server side. I have do the searching for this problem for some days already and still dont understand what is the problem.
Any help is very much appreciated.

Answer 1

^ ... continue from the comments:
First of all, if you do this, your users will hate you; secondly, that plugin is bit of a scam - all it does is uses cookies to store if user has liked a page - but if you delete your cookies, you will be locked out of the content because you won't be able to like the page again. Furthermore, it won't work without Javascript.
All the plugin does is it hooks into the "like" action when user clicks on the like button via
FB.Event.subscribe('edge.create', function(href, response){});
facebook graph api determine if user likes url