Paypal Checkout API with PHP and JavaScript

Applies to:

PayPal OAuth2.0 API v1
PayPal Checkout API v2

What?
This is an article on how to build a shopping cart and get your website to accept payments via PayPal. The below isn't a final or perfect option, it's an article to help me document and refine a process.

Why?
It appears that gone are the days when sending a payment via PayPal used to be a basic HTML form. There are probably a ton of other articles out there that document PayPal Checkout REST API v2 as well, I just want to build my own that I can understand and update.

I've chosen the API method because according to PayPal's instructions you don't have to install any additional software/framework. Ideal in my opinion.

How?
This code has been built to work alongside jQuery but it could be adapted to a pure JS solution.

High Level Recap

JavaScript » Cookie » PHP

1. Client Side JS to create Cart
2. Validate Cart with PHP on Checkout
3. Confirm Transaction with PHP and JS