PHPStan is an automated scanner, which reports code that could be improved https://phpstan.org

We already pass PHPStan level 1 since #4021

Usage:

# See https://getcomposer.org
composer install --prefer-dist
composer run-script phpstan

Now it would be nice to progress to PHPStan level 2 https://phpstan.org/user-guide/rule-levels

# Manual override of the analyse level
vendor/bin/phpstan analyse . --level 2

Help welcome, and it is fine to send multiple PRs, each solving some of the PHPStan warnings.