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Sources
This project was influenced by the PHP League OAuth2 Server and shares a lot of the same ideas.
https://github.com/thephpleague/oauth2-server
https://tools.ietf.org/html/rfc6749#section-4.4
https://www.oauth.com/oauth2-servers/access-tokens/client-credentials/
https://www.oauth.com/oauth2-servers/access-tokens/access-token-response/
https://tools.ietf.org/html/rfc6749#section-4.1
https://tools.ietf.org/html/rfc7009
https://tools.ietf.org/html/rfc7636
https://www.oauth.com/oauth2-servers/pkce/
https://www.oauth.com/oauth2-servers/pkce/authorization-request/