Description
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.