Ceit Butler
This OAuth2 tutorial is kind of great in that it kills, well, six birds with one stone. Since it centers around PHP, I'd go with learning that language. You'll also learn Composer; a PHP dependency manager, Guzzle; a PHP HTTP client, Symfony; a PHP framework, and you'll become familiar with GitHub.
The same site that offers the OAuth2 tutorial has tutorials that cover each of those pieces too. Try the PHP tutorial first.