[EVO] Tutorial: Create a Facebook fan page tab with fan gate feature

<?php
// ref: http://modxcms.com/forums/index.php?topic=63340.0
// also on FACEBOOK after creating the app > EDIT PAGE > MANAGE PERMISSIONS > SET new page as landing.
//your application secret goes here
$app_secret = "00000000000000000000"; // change to your app secret

$output = "General error";
$signed_request = $_REQUEST['signed_request'];

//read signed request
if(!$signed_request === true){
	$output = "Error no page requested";
} else {
	$data = parse_signed_request($signed_request, $app_secret);
	
	if(!$data === true){
		$output = "Error invalid signature";
	} else {
		$page = $data['page'];
		if($page == null){
			$output = "Error no page requested";
		} else {
			$fbpageid = $page['id']; //the requested page id
			$fbadmin = $page['admin']; //user is page admin
			$fbfan = $page['liked']; //user is page fan
			
			if($fbfan == "1" && !isset($_SESSION['visited'])){
				$_SESSION['visited'] = true;
				$output = $modx->documentObject['content'];
			} else {
				if ($fbfan != "1")
				{
					unset($_SESSION['visited']);
				}
				
				$docid = (isset($docid))? $docid: $modx->documentIdentifier;
				$document_tvs=$modx->getTemplateVarOutput(array('fangate'), $docid);
				$output = $document_tvs['fangate'];
			}
		}	
	}
}

return $output;

function parse_signed_request($signed_request, $secret) {
  list($encoded_sig, $payload) = explode('.', $signed_request, 2); 

  // decode the data
  $sig = base64_url_decode($encoded_sig);
  $data = json_decode(base64_url_decode($payload), true);

  if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
    error_log('Unknown algorithm. Expected HMAC-SHA256');
    return null;
  }

  // check sig
  $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
  if ($sig !== $expected_sig) {
    error_log('Bad Signed JSON signature!');
    return null;
  }

  return $data;
}

function base64_url_decode($input) {
  return base64_decode(strtr($input, '-_', '+/'));
}
?>