Dan Ackroyd maintains this interesting repo on GitHub with thoughts on why some PHP RFC's are not implemented yet. High on on my wishlist: Briefer closure syntax and Generics

There are some notes on PHP RFCs, why some were declined, and what others might need for them to be implemented. The purpose of these documents is to avoid information from being lost and to try to avoid conversations needing to be repeated multiple times on PHP internals.

https://github.com/Danack/RfcCodex