Good article. Simple to understand. I agree that OOP would make the code even more easier to follow and maintain, especially if more lines of code were to be added. Another thing, HTML is more forgiving but it is a good practice to keep track of all tags. It seems a lot of the <li> tags were not closed. That can lead to issues with some JS libraries down the road. Good thing is that modern web-browsers can 'fix' that still display the UI correctly.
RE: Computer Programming (php code for linking login/sign up page)