This function sets a callback function which is called whenever the validity of a logged in session is checked. This callback can be used to perform actions like checking the authentication source to see if the logged in user is still enabled.
The callback function will be passed two parameters, the username that successfully logged in and a reference to the Auth object. The callback function should return a boolean depending upon whether the session should continue or not. TRUE to allow the session to continue, FALSE to abort the session.
If the session is aborted by this callback the status will be set to AUTH_CALLBACK_ABORT.