Add tests

Hi Walkor,

If we have tests, will be faster to develop.

### Unit tests
Will be very beneficial but a lot of work, with the release versions (3, 4, 5).

### Functional and E2E tests
Will be easier and faster. Not necessarily with PHP.
And will work with all the versions.
We can chose a system, to all align in the efforts.

### PHP static analysers
This is really fast to use it.
We don't have test, but the static code analysers can help a lot with the code.
(phpstan, psalm, ...)

And all installed with Github actions.

Talk later !

No files, tests, or entry points are named. Start by reviewing the repository's existing test and GitHub Actions setup, then compare the proposed unit, functional/E2E, and PHP static-analysis approaches across supported versions. Done requires an agreed testing strategy and corresponding automated checks.

Written by the indexing model from the issue text.