I think Facebook just forking the language instead of helping with development gave them the kick up the ass to sort out the development process. Then it just came down to having no one actually working on the language, so they needed to create the PHP Foundation to pay people to work on it because all the major companies left it behind (Yahoo, Facebook, Zend, etc). So it's good to see it managed to survive that chaos and become a pretty good language.

The foundation is a very recent affair.

The real improvements all came from the hard work of the developers who were around during the 7.* releases who did excellent work.

Most importantly at that time we had Nikita Popov who was incredibly helpful.

Hack/HHVM definitely gave it a nice kick of motivation though.