PHP programs run only on the server. Browsers don't know how to run them.
However, browsers can run programs written in the language JavaScript. Most web apps use JS these days. For example, the site you're using now uses JS extensively. Expand and collapse sections in the lesson tree, and you're running a JavaScript program.
Buuuut there's enough in this course already. Let's not add another technology to it.
If you want to be a professional web app programmer, knowing JS is a must. That's not this course's goal, though. Soooo... let's leave JS alone.