In PHP with FusionAuth, you can pass an initial parameter through the OAuth source and have it returned on successful login. While there may not be a specific example tailored to your exact use case, you can include your parameter in the OAuth request URL like "?parmOne=Mark," and FusionAuth should be able to return it to you upon a successful login.

FusionAuth provides a flexible system for handling OAuth parameters, and you can adapt the PHP example you mentioned to suit your needs. For a beginner-friendly example, you can start with the basic PHP integration and customize it to include your desired parameters.

FusionAuth Cloud does not have a strict user count limit for Medium and Large size plans. The 200k and 750k user limits are recommendations based on the expected login and registration volume. If you need to support more users, you can contact FusionAuth support for assistance.

The document you linked to does not mention a user count limit because there is none. The recommendations are simply a way to help you choose the right plan for your needs.

If you are unsure how many users you will need to support, you can start with a smaller plan and upgrade as needed. You can also contact FusionAuth support for help choosing the right plan.