Get rid of PHP 8.1 deprecations warnings

[joworeiter]

Since the upgrade to PHP 8.1 we get a ton of deprecation warnings.
This could easily be fixed by adding return types, which are only available since 7.1, so I would advocate to set the min. php version at least to php 7.1 and fix the warnings by adding proper return types, where possible or supress them by adding the willChange-Attribute.

If someone could review my last pull request, I would be deligthed to serve an other one for this ones.

[rjjakes]

@joworeiter I've merged your previous PR. You can raise a new PR for this issue.