Facebook API - Detect if a user like's the app's page

fb_like.php

<?php
// Get Signed Request	
$signed_request = $_REQUEST['signed_request'];
  	
// Parse Signed Request
function parsePageSignedRequest() {
    if(isset($_REQUEST['signed_request'])) {
        $encoded_sig = null;
        $payload = null;
	list($encoded_sig, $payload) = explode('.', $_REQUEST['signed_request'], 2);
	$sig = base64_decode(strtr($encoded_sig, '-_', '+/'));
	$data = json_decode(base64_decode(strtr($payload, '-_', '+/'), true));      

        return $data;
    }
    return false;
}

// Display content depending on if the user likes the page or not.
if($signed_request = parsePageSignedRequest()) {
    if($signed_request->page->liked) {
    	echo "YOU LIKE IT!";
    } else {
    	echo "LIKE TO VIEW CONTENT";
    }
}
?>