That is of course a bit of theoretical understanding; eCommerce. Since you have already learnt HTML, CSS, JavaScript and PHP with MySQL. I would suggest that you build up the concept of an eCommerce website. It is simply a web store, where your database records hold the information about:
1. Customers ↔ Sellers
2. Items to be sold.
3. Items in the cart.
4. Payment information and checkout systems; etc.
It is just the database that would contain the tables for these records. A shopping cart is just the record of "item to be purchased" in the database. Once user continues to the next step, that record is added to "Purchased" table and the items are delivered. For more on this, please read the article that I have associated below. I wrote that article for ASP.NET and SQL Server, you can use your concepts of PHP and MySQL.
The tough part comes, when you have to integrate the payment gateway. You should not purchase the licenses and agreements for a personal payment gateway. They are very expensive and require a lot of complex tasks to be done. So, consider a third-party payment gateway, find the one that suits your needs and budget and then continue. Integrate it to your web application, you're good to go! I don't suggest any books, because then you have to stick to
1. The standards of that book.
2. Philosophy of that author, not the concept or field.
3. Framework being used; otherwise you get too many errors.
4. Tools provided and concepts given.
5. Being the same novice in the framework.
This is why, I don't recommend books. Instead, I recommend online resources to learn the framework or the concept itself. Find the Wikipedia article below, read it and understand the term of eCommerce. Once you get the concept, you will be able to
create it from scratch.
Creating a shop website in ASP.NET[
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E-commerce - Wikipedia, the free encyclopedia[
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