Create HTML form with PHP Class by tisuchi. This section would be very simple task for OOP with PHP developer. Although it could be very good foundation for beginner of Object Oriented Programming in ...

3. http redirect to where I want to send the user after that based on form results. what are my options to remove the intermediate page but still have the server-side code run? is there some sort of ...

send-email.php is the only endpoint: it takes the POST, filters, and sends. All credentials live in a .env file at the project root (see .env.example) — never in the source tree, never in Git.