Facebook hasn't used PHP for a very long time. It's been using Hack for over 12 years, which was initially based off PHP but it's deviated quite significantly now. It's now nearly entirely statically-typed, and has a bunch of features that PHP doesn't have. IO is fully asynchronous using async/await syntax (like C# or JavaScript), functions can be marked as pure, and a bunch of other things useful in large codebases.

It still uses the same request model as PHP, since that's genuinely useful for web requests. It's easier to avoid memory leaks if each request is entirely self-contained and there's no global state persisted between requests (other than APC and opcode caches)