Didn't they start with it and migrate away?

They use something called HHVM, which is a PHP compiler.

http://hhvm.com/

And Hack, their language for HHVM that's basically "static types bolted on to PHP".

http://hacklang.org

An aptly named language if there ever was one.

Hack is more than that. Hack solves every little stupid mistake PHP has as a language. It is a good language developed by smart people, has rules to avoid pitfalls and rules to make things more certain. It retains only the syntax of PHP, but in strict mode, hack can be compared to any other good language.