Set a callback function to be run when a single-signout request is
received. 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 a
session-id to the logout request in order to support single-signout in
applications that manage their own sessions (rather than letting phpCAS
start and destroy the session).