Earlier this year I did a PoC for using Keycloak from a PHP application, and contributed some code[1] to add a provider for PHP League's OAuth 2.0 Client[2].

[1] https://github.com/stevenmaguire/oauth2-keycloak 

[2] https://github.com/thephpleague/oauth2-client

Please have a look if it suits your needs although it will probably need some additional work depending on which features you want to use.

Best regards,

Thomas