It's an alternative PHP runtime. It no longer compiles PHP to C++ (it's predecessor hphpc), instead it uses a JIT compiler which gives it better performance ove

It's an alternative PHP runtime. It no longer compiles PHP to C++ (it's predecessor hphpc), instead it uses a JIT compiler which gives it better performance over standard PHP, without requiring a compilation step. It's mostly compatible with the standard PHP syntax/stdlib. It offers an alternative syntax that allows you to give return types of functions, which can give you further speed improvements. There's also a few additional features (for example, it supports similar hinting to the recent ArrayOf RFC). It's worth checking out, but probably not worth using in production yet - they're working on getting the top 20 frameworks to pass their unit tests in HHVM[0], so there's a good chance your existing apps will run. [0]: http://www.hhvm.com/blog/2813/we-are-the-98-5-and-the-16 http://www.hhvm.com/blog/2813/we-are-the-98-5-and-the-16