Yesterday, Facebook announced Hack, a new language that also runs on HHVM. It’s like a “PHP++” — it adds optional static typing, generics, and a bunch of other enhancements and conveniences to PHP.

Unlike HHVM, adopting Hack is a huge risk. HHVM was great because you could switch to it and switch away from it freely, with almost no effort (especially to switch away). You were still writing PHP. But once you convert a file to Hack and use any of its new features, it’s no longer valid PHP, so you must always use Hack and HHVM from that point forward (or undertake an expensive rewrite).Hack Isn’t PHP – Marco.org

Be careful not to be locked-in.