After I originally posted, I saw that Hack also includes async support. I haven't tested it yet, but that is a major change from PHP, I could see that as being very useful in the future assuming it works well (it looks like they just implemented it in 3.5).
As some may know, PHP is a synchronous language, each line of code is executed and it waits to finish execution before moving on to the next line. This makes it simple to use, but somewhat inefficient if you have something like a remote CURL request, SSH/FTP login, or a long database transaction. PHP could be doing other things while it waits for a response.
So from my understanding, you use the async keyword on the function, run it, perform other code, then when you get to the point when you need the response from the async function, you can use await.