Some things are wrong
I’m giving this 3 out of 5 because it’s a promising start but needs improvement. I kind of like the UI. I read about others saying they didn’t like it. It’s simple and clean. Works for me. My main issue is that some of the things are just wrong. I’ve been a PHP programming for more years than I can remember but I am far from an expert. I use apps like this as they are good refreshers for me. Something productive to do during my restroom breaks. Always good to go over the basics again and again. I noticed a few errors. Some of the things being taught will not work as explained. You might consider building in a way for people to report errors in the app so you can check and fix them. Or just so they can alert other users and allow people to discuss how it should have been written.