Continuing work started with PR #4016 and PR #4026. Improve documentation within program by making explicit what types of values are allowed for variables described as "mixed". In order to avoid broken functionality, this is done mainly through doc-blocks. This will get us closer to Phpstan Level 9, but many changes will be needed before we can consider that.
This change has more executable code changes than its predecessor. I will wait longer than normal before merging it to allow for additional testing.
While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
oleibman
Adrien Crivelli
Mark Baker