Facebook SDK，如何将用户名放入墙贴？

I got this code and working fine

    $access_token = $facebook->getAccessToken();
             $vars = array(
                  'caption' => 'Caption message',
                  'message' => 'I need help',
                  'name' => 'I need Help',
                  'link' => 'http://www.google.com/',
                  'description' => 'description',
                  'picture' => ''
                            );

But I want to replace the message or the name in to like : " (name of user) Need Help...

I don't know much about the API, but there is a method for

$facebook->getUser();

http://developers.facebook.com/docs/reference/php/facebook-getUser/

edit: there's a better example here:

facebook application get the username using php api