PHPUnit is failing because it's trying to run PHPUnit 11 which requires PHP >= 8.2, but that's not what it says in the composer.lock (or even componser.json).

I will need to upgrade the actions runner as described here: php-actions/phpunit#54

@g105b PHP core maintainer here. I fixed this here: php/php-src#14206
It was already fixed on the PHP 8.4 development branch because I rewrote the entire ParentNode implementation when implementing opt-in DOM spec-compliance. So I just backported that fix.
Please just submit or ping us when you encounter something like this, otherwise stuff like that never gets fixed or fixed only after a long time. To give an idea: patching this only took 15 minutes from reading this PR until submitting my PR, so even if you just have code it's often easy to fix these issues. It's just that we have to be aware of them in order to be able to fix them.

@nielsdos hey, thank you for your message. I will submit an upstream issue whenever something like this comes up, but I've never felt confident to do it. I see now that PHP has an issue tracker on Github, so I guess it's just as simple as opening an issue there? That's a lot LOT easier than before, when it was on bugs.php.net.

Thanks for the fix, I'll test it and see if I can remove more of the workarounds in this library.

Yep it's just a matter of opening a GH issue. It doesn't even have to be much, just code is enough.
It's worth noting that for PHP 8.4 I have introduced new classes that fix a lot of crap in the DOM extension, full story in the RFC.