Document code-style for enums and class constants

[derrabus]

This PR adds some naming conventions.

First of all, class constants. We seem to have an unwritten rule on how class constants should be named. I've made that rule explicit.

Secondly, it adds enumerations to the mix. Those are a new language element introduced in PHP 8.1. Our codebase does not have any enums yet (apart from test fixtures), so I think we should discuss the options before the first PR adding an enum to Symfony's public API arrives.

I took inspiration from the example code from the RFC. The most complete example is probably this one:

enum UserStatus: string
{
  case Pending = 'P';
  case Active = 'A';
  case Suspended = 'S';
  case CanceledByUser = 'C';
}

I propose to adopt the naming conventions used by the RFC because they seem reasonable to me.

Fixes #16538, symfony/symfony#45516.

[nicolas-grekas]

Works for me. I would add that attributes should be in an Attribute namespace.

[wouterj]

I would add that attributes should be in an Attribute namespace.

In that case, let's move Route from Annotations to Attribute :)

[derrabus]

I would add that attributes should be in an Attribute namespace.

In that case, let's move Route from Annotations to Attribute :)

Yeah, and then there are validation constraints that also violate the rule. We need to document a couple of exceptions to that rule which is why I'd like to separate attribute namespacing from this discussion, if that's okay for you.

[nicolas-grekas]

Yeah, and then there are validation constraints that also violate the rule. We need to document a couple of exceptions to that rule which is why I'd like to separate attribute namespacing from this discussion, if that's okay for you.

Works for me also :)
A fallback convention would then be that attributes should be in a dedicated namespace. That would fit Constraints and Annotation(s) namespaces.

[stof]

The case of Constraints is special, because they are not only attributes. They are the API of the component directly.

[javiereguiluz]

Thank you Alexander.