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.
Things still being discussed
These need to be summarised.
Ini settings - why app level settings are kind of okay, but library level ones are bad.
Null short-circuiting - https://wiki.php.net/rfc/nullsafe_calls
Pipe-operator - https://wiki.php.net/rfc/pipe-operator
type number = float | int;
Ideas that overcame their challenges
Please read the contributing guidelines before writing your 4,000 word novelella on why some RFC really should pass.
I reserve the right to be as opinionated as I feel like when commenting on RFCs; I'm not going to attempt to stay 100% neutral when talking about RFCs that I consider to be either dumb or bad for PHP.