Example of Okta integration for legacy PHP website with JIT

I was looking for some guides, instructions or examples on how to integrate the Okta with the existing PHP website and existing user base. So, basically, user migration is required (“just-in-time”).

As I understand, authentication API can be used to check if the user exists in Okta, but how can I authenticate the user with it?

Are there any guides of building JIT migration in PHP? I see only this article which is just an overview: https://developer.okta.com/blog/2019/02/15/user-migration-the-definitive-guide.