This change introduces PHPStan static analysis configured at [https://phpstan.org/user-guide/rule-levels rule level 0], which includes: "basic checks, unknown classes, unknown functions, unknown methods called on `$this`, wrong number of arguments passed to those methods and functions, always undefined variables". Contributors may elect for a higher PHPStan rule level by creating a `phpstan.neon` which overrides `phpstan.neon.dist`.
* Fix various PHPStan level 0 errors by adding `@phpstan-ignore` comments, updating PHPDoc types, and adding missing return values.
* Remove existing `@phpstan-ignore` comments that are now obsolete or inapplicable for level 0.
* Add a new GitHub Actions workflow for PHPStan Static Analysis. Reports are currently provided as warnings with inline annotations in pull requests and do not fail the build.
* Add a `phpstan` Grunt task and include it in the `precommit:php` task to run before `phpunit`.
* Introduce a `typecheck:php` npm script and a `composer phpstan` script to run analysis in local development environments.
* Add documentation for PHPStan usage in `tests/phpstan/README.md`.
Developed in https://github.com/WordPress/wordpress-develop/pull/10419
Props justlevine, westonruter, johnbillion, desrosj, SirLouen, dmsnell, oglekler, joehoyle, jorbin.
See #64238, #63268, #52217, #51423.
Fixes#61175.
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