If you squint, Meta. A large part of the codebase is in Hack. It started its life as PHP and still has a lot of the heritage there. Slack also uses Hack.
+1 to this as a former Meta employee although I don't think you need to squint. That being said, if you're good, you should realize that the code is less important than the grasp of concepts, patterns, data structures, algorithms.
As someone who has wrote PHP as part of a job in the 90s, the 00s, and the 10s, and at least seen some Hack at Facebook (I think I wrote a tiny amount of code in Hack, but it wasn't in the main repo), Hack code, especially the idiomatic code in the main repo, is pretty detached from PHP.
Yes, there's $'s everywhere. Otherwise, it looks more like a Java codebase than a PHP code base, IMHO. Although, maybe I've mostly seen PHP code bases where there's less abstraction/OO::Explosion, I've never worked long term on PHP code with an opensource framework, and I didn't like what I saw when I had to look at things in those frameworks. IMHO, that makes for too much hidden logic, and makes it hard to understand what's happening, and why it takes 20-50ms to output a hello world.
Full disclosure: I work for Meta.