Facebook is migrating nearly all of it’s codebase from PHP to Hack!. Hack is a new programming language for HHVM (Hip Hop Virtual Machine).
Quite similar to and derived from PHP, it was needed to speed up generation of web page. Also currently PHP has a lot of security loopholes which do not die down even after the code is changed into C++, hence facbook was thinking of changing it.
Click here for new of the change of code.
Click here to know more about HHVM.