Stricter PHPStan levels

[eucyt]

PHPStan allows you to choose the type of warnings based on levels.
https://phpstan.org/user-guide/rule-levels

It is currently set to level 5 (the maximum is 10). Let's make it stricter and refactor the code for better quality.

[eucyt]

I have raised the level to 8. Increasing it further would lead to more redundant code written solely for PhpStan, so I think it's best to stop here for now. Therefore, I will close this issue.