From Facebook Developers Wiki

Trying to write adesktop app, but method auth_createToken() is referenced in comments for auth_getSession()but doesn't actually exist in facebookapi_php5_restlib.php or any other faceBook PHP library. Has this been deprecated or is it not supported in PHP?

Ping/Bump etc. This hasn't been answered yet and I'm having the same problem.

In http://forum.developers.facebook.com/viewtopic.php?pid=20785 I find this solution

public function auth_createToken() {

   $result = $this->call_method('facebook.auth.createToken', array('api_key'=>$this->api_key));
   return $result;

}