PHP 8.1-8.3 code hardening + PHP 8.4 version bump

[hopeseekr]

All copyright and IP rights transferred from PHP Experts, Inc., to @brendt brent.roose@gmail.com.

This is a corporate code donation.

Fixes #165.

[brendt]

I don't think this is the way to go: added readonly to classes is a breaking change. Furthermore, I personally don't want to use #[Override]: https://stitcher.io/blog/override-in-php-83

[hopeseekr]

Is there an API server for this actual project?

https://highlight.phpexperts.pro/ provides a simple API server for this project currently, but because this PR and others from me have been rejected, it seems that in the near future, it will start serving content from the forked version instead...

This API service is accessible to any language, evne plain HTML + a single JS and CSS include. It's the only way I could get it on my non-PHP 8.3 websites (almost all of them).