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I asked a similar question recently, one of the maintainers was nice enough to reply with some info. Perhaps this sheds some light: https://news.ycombinator.com/item?id=23116754

I think the point stands doubly strong in case of Facebook. They're a PHP shop after all.

PHP doesn't run in the browser...

Sure, but using it in the backend tells us that there is no language meritocracy at FB.

It is.