This repo contains various examples of authenticating with VATSIM authentication (https://auth.vatsim.net/)
The PHP example is written in pure PHP with no add-on libraries used. To use this example, you'll need to set the correct values in secrets.php with the ones you've obtained from VATSIM. By default this example uses the VATSIM auth-dev server, which is a testbed for those implementing VATSIM authentication.
More examples in other languages may follow.