Fix PHPStan L1 error "Variable $foo in/on something() is never defined."

Problem/Motivation

Amongst the errors when running PHPStan on level 1 is: Variable $foo in/on something() is never defined.

This issue exists to fix all of those and remove them from the suppressions in the current baseline core/phpstan-baseline.neon.

Steps to reproduce

- Find six of them in being suppressed in the current baseline core/phpstan-baseline.neon.

Proposed resolution

- Solve all of the the above mentioned reported errors.
- Remove them from the baseline.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet