I considered your reasoning but seeing it onscreen helps :) I'm leaning your way, but here were my thoughts:

Aliases make an API worse, forcing decision points and investigation: Whenever I see autocomplete choices for `sanitize_string` and `sanitise_string` I wonder "which one is the 'right' one? Wait, is there any code differences?"

API naming inconsistency is certainly bothersome, but most PHP devs appreciate having legacy names Just Work for a long time; it makes PHP software durable. It bothers me more that in some code I see `count` and elsewhere `sizeof`.