"PHP strives to maintain backwards compatibility", I'll have to say I've seen enough evidence to the contrary. Even basic object-oriented metaprogramming have been radically changed in minor point/patch releases. Stuff like "get_class_vars()" changing from returning private vars, to not returning private vars, etc. (edit: between PHP v5.2.6 and v5.2.7 if I remember correctly)
Although true, I would say that's kind of an "abuse". I find many PHP functions have changed over versions to give more predictable output and validate expected input more stringently. If backwards compatibility wasn't a concern, they would attempt to standardize the lib in obvious incompatible ways.
Well, backwards compability is kind of a binary thing, isn't it. Either you're aiming for backwards compability and you can be sure your legacy code will keep running, or you don't. In fact it would be more helpful to throw fatal errors because a method signature changed, than start returning different and unexpected values which can break data integrity in subtle ways.
