I think you can use reflection [1] to block the hooks from using & in their parameter lists. This way the clients must use & to pass a reference. This is definitely possible for function/method callbacks [2], but I'm not sure about 5.3 closures [3].
David [1] http://www.php.net/manual/en/reflectionparameter.ispassedbyreference.php [2] http://www.php.net/manual/en/language.pseudo-types.php#language.types.callback [3] http://www.php.net/manual/en/functions.anonymous.php
