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Isn’t a significant amount of the codebase in Hack, a PHP derivative?

C++ is the most popular language at Facebook, I know that one for sure. They used to run PHP on HipHop VM which was written in C++, but now they transpile PHP to C++.

The transpiler is HipHop. They discontinued that in favor of HHVM, which does JIT compilation instead. More info: https://hhvm.com/

EDIT: apparently, though, HHVM stopped supporting PHP itself last month; now it only supports Hack. I'm not familiar enough with Hack to know how much it actually deviates from / improves upon PHP.

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