Facebook PHP SDK, how to define callback URL

I have written following script, which checks user's session and displays login/logout links accordingly. Script works, but the problem is, I don't know how to set callback URL, there is no field(or at least I can't find one) at my FB app settings page. Any ideas?

$facebook = new Facebook(array(
  'appId' => 'xxx',
  'secret' => 'xxx',
  'cookie' => true,
));

$session = $facebook->getSession();

$me = null;

if ($session) {
  try {
    $uid = $facebook->getUser();
    $me = $facebook->api('/me');
  } catch (开发者_StackOverflow中文版FacebookApiException $e) {
    error_log($e);
  }
}

?>
<?php if ($me): ?>
  <a href="<?php echo $facebook->getLogoutUrl(); ?>">Logout</a>
<?php else: ?>
  <a href="<?php echo $facebook->getLoginUrl(array('req_perms'=>"email,publish_stream")); ?>">Login</a>
<?php endif; ?>

The parameter is called "next". So you do it like this:

<a href="<?php echo $facebook->getLoginUrl(array('req_perms'=>"email,publish_stream",'next'=>"welcome.html")); ?>">Login</a>

Change "welcome.html" to wherever you want your user to land on your server after logging in.

IN the facebook php sdk file , about line 418

 public function getLoginUrl($params=array()) {
    $currentUrl = $this->getCurrentUrl();
    return $this->getUrl(
      'www',
      'login.php',
      array_merge(array(
        'api_key'         => $this->getAppId(),
        'cancel_url'      => $currentUrl,
        'display'         => 'page',
        'fbconnect'       => 1,
        'next'            => $currentUrl,
        'return_session'  => 1,
        'session_version' => 3,
        'v'               => '1.0',
      ), $params)
    );
  }

and 445

 public function getLogoutUrl($params=array()) {
    return $this->getUrl(
      'www',
      'logout.php',
      array_merge(array(
        'next'         => 'http://yoururl.com/?logout=1', // or $currentUrl the same page who make the callback , and destroy session if it.
        'access_token' => $this->getAccessToken(),
      ), $params)
    );
  }