PHP is used for a subset of development at Facebook. I'm not sure what the split is, but I would not be surprised if PHP is the primary language used by less than 60% of Software Engineers at Facebook. The rest code in C++, Java, Python, and then a bunch of other languages that are less used.

The PHP development environment at Facebook is unlike that found at any other company I'm aware of, and common PHP pitfalls and legacy code are actively "linted" out of the system (as an automated part of the code review process). It still wouldn't be my choice of language to use outside of Facebook, but I have no issue working on the PHP code base here (although I rarely venture into it).

Over time, much logic and processing has been offloaded to backend services, with PHP increasingly playing a (largely parallelised) dispatch, post-process, and render role. That will likely continue.

With all that said, it is questionable whether it would be an investment to move off of it - which language/environment would you think a good target would be, and why?