At work, we've spent a lot of time trying to figure out the best way to handling linting Pull Requests.
We had some success with mergeatron + Jenkins for a long time, but eventually started to run into problems (like it crashing the server it was running on and being super slow).
We spent some time playing with GitHub Actions which seem really interesting. Unfortunately, at the moment, it's still kind of the Wild West in the Actions Marketplace and the documentation for writing your own actions is spotty at-best.
We also played with different Continuous Integration tools like TravisCI and CircleCI, both of which are very full featured but also expensive. They both seem like very heavy solutions for what we're trying to do.
We're a small team so all we really need is something that runs
phpcs on any changed files in a commit.
Here's a sample project that can easily be added to a project of your own:
We're definitely not ruling out other tools, long-term. But the above should be a decent process for now