If your client-side code doesn't even require server-side validation, it does make it difficult to trust or believe your sandbox or app is secure.

As I said, it's a cool idea, but if you want developers to recommend this to people starting out, you need to make those developers confident that they're giving people something well-written and secure.

Edit: Well, cheers to your edits! As someone who is often helping people with learning PHP, I will definitely keep an eye on this project as something to recommend :)