Well, as far as I am aware of... HACK is supposed to be a better version of PHP, so it should be object oriented and uses of HACK should be very similar to the uses of PHP.
Here is something I found:
Hack is a programming language for the HipHop Virtual Machine (HHVM) invented by Facebook. The language is open source.
Hack can be seen as a new version of PHP that also runs on the HHVM, but it allows programmers to use both dynamic typing and static typing. This is what is called gradual typing, which has been mostly seen as an academic exercise; as described by Facebook, it is the first to bring gradual typing to a "real, industrial strength" language.