The OpenID provider I use, Stalwart, does not support client_secret_basic, and jumbojett/openid-connect-php defaults to client_secret_basic being enabled, even if it is not enabled. See issue here. This results in openid login failing with invalid_client against a provider that does not support client_secret_basic.

openid-connect-php does offer a method to disable client_secret_basic, located here.