[RFC] [Discussion] Class Friendship in PHP

Hi! Almost two years ago, I opened discussion regarding my proposal to support class friendship through a new keyword, `friend`. For a number of reasons (mostly work-related; some personal) I didn't have time to move the process forward. There were several hot-topic discussions going through the mailing list at the time that I believed deserved more attention. I feel that my RFC was ill-timed, perhaps. That said, we're almost two years into the future! I would like to re-open discussion of my RFC and get it to vote appropriately soon. In the previous discussion, there was on-going work to introduce package-private classes by Guilherme. As far as I can tell that work is stalled, pending fundamental changes to how namespaces are implemented in PHP (https://github.com/php/php-src/pull/947#issuecomment-224912697). Like many others, I am very excited for the possibility of package-private classes or namespace visibility features of any kind. Even so, my personal vision of that landscape includes class friendship as a unique, explicit and concise expression of tight-coupling between object-oriented collaborators. I do not feel these efforts are either-or. I believe they both contribute to a more expressive means of specifying the relationship between collaborators with regard to visibility. The RFC is at: https://wiki.php.net/rfc/friend-classes I need to rebase my implementation against `master` and send a PR. I will have that done before voting begins as it includes several test cases and I would appreciate additional perspectives to catch edge-cases I may not have thought of. The implementation was quite straight-forward. Thanks!