I get why they want to get rid of references. If I would be designing PHP from scratch, that's probably the first thing I'd do - most hard to fix bugs in the engine and most security woes of features such as unserialize() stem from references. Unfortunately, TONS of code uses them, and there are some data structure things that would not work without it unless alternative solution is provided. So it could not happen in PHP. But it'd be interesting to see how it works for Hack.