PHP is great for being able to easily create web apps that work. The problem is that it also easy to create code with major vulnerabilities that is a mess to read and try to audit. Many times PHP is used by desktop support technicians to create apps to track department inventory, or work orders that after a few years get coopted by the rest of the company and are being used at a scale never envisioned when the technician was copy/ pasting from a dummies book to make the code work.
By turning a file into an array you can work with the data within it as you would a standard array.
Cookies allow you to set variables that are persistent between multiple visits to a website.