Would it rock? Nah. It would merely make it less horrible. It lacks fundamental elegance and it lacks advanced capabilities. Even if fixed, it would still only be clinging to the crown of mediocracy.

But that is fine. Not all languages can or should be mind bending or define its own paradigm. So yeah, if all its problems was somehow in all unlikelyness fixed, it would be fine. It would be fine, but it would in no way "rock".

It would be a completely different language.

Yes I realise that

In that case, why not just use any of the existing developed alternatives rather than developing an entire new PHP replacement from scratch?

Because the alternatives suck in different ways.