Support PHP 8+ and update test suite

[SimonFrings]

As pointed out by @mahdyaslami in #88, this project us currently not compatible with the newest PHP versions, in particular PHP 8+. Supporting the newest PHP versions requires a few adjustments, but most work will go into making the new changes compatible across all supported PHP versions (PHP 5.4+). At the same time, we also need to update our test suite to use the latest PHPUnit versions for newer PHP.

We're currently not sure when we'll take a look at this, if this is needed urgently, feel free to reach out to us or help us prioritizing this by becoming as sponsor ❤️