PHP8 Support

[flavioheleno]

This is:

- [ ] a bugfix
- [x] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

This change is discussed in more details in #1552

[MarkBaker]

There is actually a separate branch being used for this work at the moment. PHP8-Testing currently includes testing with PHPUnit 9.4 against the PHP8 nightly build. Code is currently has 7 errors, and is failing 1 test. If you do want to work on this, then it might be worthwhile working from that branch rather than against master.

The biggest remaining problem is handling allow_failure for PHP8, because of the way that we're using job stages rather than a simple grid; I've yet to identify how we can implement that to allow failures for testing against PHP8, while still allowing the other stages to run, which is why it's on a separate branch at the moment.

[MarkBaker]

And once we have all tests passing with PHP8; the next big step is going through all public methods ensuring that argument names are meaningful, in preparation for PHP8's named arguments changes. But I'd certainly welcome all the help I can get with that change; and meaningful argument names can be done in a fork from master

[PowerKiKi]

Superseded by #1675