Beginner's question concerning php

Now, I want to learn php language in order to create a customer area on my website. So I created a Simple page with inputs in html and css and I would like to place my php commands (registration of the username and password, then, client authentication). To start, I would first have to know where to save my future file.php, I mean at what level? Maybe I need to save it to my folder “simple-pages”? Or in other one?

Thank you in advance for your advice

You might want to look at the Guest User plugin, which will handle user authentication.

Thank you for your answer. This is precisely the plugin that I installed! As well as another open source plugin that serves as a shopping cart.