Freek Lijten has a recent post looking at some of the types of discussions that happen on the php-internals mailing list.

The internals list is the place to be to hear about the current state of PHP. It is one of PHP's many mailing lists, but this is the one where (core) developers discuss new features, current bugs and wild ideas. If you want to keep up with things it is a good idea to sign up, it is not an extremely high volume list and if you ignore the noise it is quite informative. In this article I would like to share examples of stuff typically discussed on the list.

He mentions feature requests in general and, more specifically things like traits support (multiple inheritance), array dereferencing, callable arrays and the debate over the short array syntax.