PHPStan is a great command line tool for looking at how your PHP code will run without actually running it. It's great for finding potential bugs that you wouldn't have otherwise discovered using other tools or through unit testing.
With regards to Drupal projects there is a little problem in that PHPStan doesn't know how to interpret Drupal plugins, entities, controllers or all the other Drupal architecture that goes into a Drupal module. For this reason, if you try to run PHPStan on your module code you'll find that it produces a lot of errors regarding missing objects or incorrect parameters.
Thankfully, it's possible to easily teach PHPStan about Drupal and make the tool more useful when writing Drupal code. First we need to install it.
Read more here: https://www.hashbangcode.com/article/drupal-9-running-phpstan-drupal-custom-modules