The callback function will be passed a $logoutTicket as its first
parameter, followed by any $additionalArgs you pass. The $logoutTicket
parameter is an opaque string that can be used to map the session-id to
logout request in order to support single-signout in applications that
manage their own sessions (rather than letting phpCAS start the session).
phpCAS::forceAuthentication() will always exit and forward client unless
they are already authenticated. To perform an action at the moment the user
logs in (such as registering an account, performing logging, etc), register
a callback function here.