I am an amateur Web Developer. I am making websites as a hobby and I usually make simple or static websites. Recently, I found a client that wants an E-commerce. I do know how to code on php but I turned it down due to lack of knowledge on ecommerce sites and handling credit cards. So now I am asking for help in gaining more understanding in this field. I tried searching off the internet but I'm having troubles finding what I really need. Most of them are just guides that lead to merchant sites such as shopify or whatever, if you guys could give me a link, that would be really great!

First off, I do know how to make a shopping cart or check out page. The part where I'm having troubles is how do I process the credit cards? I'm guessing that you need to have some kind of third party payment processor or some sort. How does that work?