Pass-by-reference not allowed in hook functions.

Hook functions that are called from module_invoke() or module_invoke_all() should not expect their parameters to be passed by reference.

See for example the notes for user_module_invoke():

We cannot use module_invoke() for this, because the arguments need to be passed by reference.

Pass-by-reference does not work for functions called by call_user_func_array(). PHP 5.2 silently ignored this coding error, but PHP 5.3 issues a warning and replaces the function result with FALSE.