Improve recognition of constants in PHP

[ghost]

Resolves #1687.

[RunDevelopment]

I don't think it's a good idea to say that any word which isn't highlighted as anything else should be a constant.

Let's say there is a feature of PHP which Prism doesn't support (yet). All of the keywords/names related to this feature will be highlighted as constants.
Today, these features are type declarations, group use declarations, return types, and type casting just to name a few. These are only partially supported or not at all.

I encourage you to play around with the examples on the PHP.net page and see for yourself what would be highlighted as constants.

But apart from that, isn't it a naming convention to not use lowercase characters for constants?
Even the page you linked to follows it.

[ghost]

All of the keywords/names related to this feature will be highlighted as constants.

That’s the way PHP behaves in some circumstances, such as echo X. However, I shall agree that there are other contexts, such as function f($x): type {}.

Prism definitely shouldn’t mark unsupported elements as constants.

OK, let’s stick with PSR‑1, not language itself, i. e., underscore as a word separator only, no lower­‑case letters. Should we have single­‑letter constants (X), then? I think we should. It doesn’t affect type declarations, group use declarations, etc.

[RunDevelopment]

Should we have single­‑letter constants (X), then?

Sure!

[RunDevelopment]

Thank you for contributing!

[ghost]

Thank you!

But it’s sad that the PR was merged before Travis CI has finished building :(

[RunDevelopment]

Don't worry about that.
When you look at the detail, you'll see that all tests passed on 2/4 Node version and the other 2 couldn't find this PR (probably because it was merged already).

[ghost]

OK, thank you!