Turns WordPress or WordPress MU into a CAS single sign-on authenticator.
This hook allows for the insertion of code after login has successfully completed and just before the ticket creation. One common use of this hook is to fill out the $_SESSION variable with site/user specific information.
This filter (executed in a successful ticket validation in /cas/validate) is used to override the user identifier returned in the cas:authenticationSuccess XML response. By default, the value returned is the $user_ID of the authenticated user. Using this filter, that value can be altered to whatever suits your implementation.