Laravel custom validation - get parameters

As a 4th the whole validator is passed to the closure you define with extends. You can use that to get the all data which is validated:

Validator::extend('event_sponsor', function ($attribute, $value, $parameters, $validator) {
    $sponsor_id = array_get($validator->getData(), $parameters[0], null);
    // ...
});

By the way I'm using array_get here to avoid any errors if the passed input name doesn't exist.