Facebook iFrame check

gistfile1.php

<?php
// Access with ?dev=true for debugging
$signed_request = isset($_POST['signed_request']) ? $_POST['signed_request'] : false;
$debug = isset($_GET['dev']);
$secret = 'APP_SECRET';

if(!$signed_request && !$debug) {

	die('No direct access.');
	
} else if ($signed_request) {
	
	list($encoded_sig, $payload) = explode('.', $signed_request, 2);
	$sig = base64_url_decode($encoded_sig);
	$data = json_decode(base64_url_decode($payload), true);
	$expected_sig = hash_hmac('sha256' , $payload, $secret, $raw = true);
	
	if (strtoupper($data['algorithm']) !== 'HMAC-SHA256' || $sig !== $expected_sig) {
		die('Invalid request.');
	}
	
}

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