FB php detect expired access_token from error.

<?php
session_start();
require './src/facebook.php';
$facebook = new Facebook(array(
  'appId'  => 'YourAppId',
  'secret' => 'YourAppSecret',
  'cookie' => true, // enable optional cookie support
));
try {
$user = $facebook->getUser();
} catch (FacebookApiException $e) {
$user = null;
}
if($user){
try {
$me = $facebook->api('/me?fields=id,name,first_name,permissions');
}catch (FacebookApiException $e){
$thiserrorResult = $e->getResult();
    if(isset($thiserrorResult[error][code]) && $thiserrorResult[error][code]=='2500' ){
    // we need to reauth
    $facebook->destroySession();
    // do redirect here, or show login button
    }
}
}
?>
<!DOCTYPE html>
<html>
<head><title></title></head>
<body>
Add javascript sdk here.
</body>
</html>