How to getSignedRequest

$app_id = "apid";
$app_secret = "appsecret";
$facebook = new Facebook(array(
        'appId' => $app_id,
        'secret' => $app_secret,
        'cookie' => true
));


$signed_request = $facebook->getSignedRequest();

$like_status = $signed_request["page"]["liked"];

// If a fan is on your page
if ($like_status) {
$a = file_get_contents("https://mysite/likepage");
echo ($a);
} else {
// If a non-fan is on your page
$a = file_get_contents("https://mysite.com/reveal");
echo ($a);
}