The container.php send the address to the submit.php page through a POST request, so obviously in the submit.php file; that's where you receive the address through $_POST["address"] and do all the logical back-end part of the code (verify address and insert into db).

I suggest that you see/read some PHP tutorial to understand what is happening there. That's the most basic stuff, and it's hard to make other people clear that part to you (and write the code) since they don't even know how your project is structured and what you are trying to do (e.g: you never told me your form was in a .php file, and the logical part, which receives the data, was in another one. So how would I guess?).