I can think of a couple of practical uses, the biggest one that comes to mind is helping places like CodeSchool do in browser PHP tutorials with the code actually executing. So definite practical uses :)

Assuming you have strong convictions about using php instead of JavaScript, it could be very useful.

I'm actually using a similar tool (js_of_ocaml [1]) at work. It's great: we can take our product and package it for node or let people use it in the browser directly. And we can also have a native compiled version if performance is needed.

[1]: http://ocsigen.org/js_of_ocaml/

So if you have an almost religiously php-oriented shop working on something that isn't a web app, I could certainly see this being useful. That's exactly what the company I'm at is, except with OCaml instead of php. Of course, I can't actually imagine any company like that in php :P. (How many people use php for something that isn't server-side code?)

But the core idea is useful and awesome. And it's certainly very cool for a personal side-project!

Actually, I stumbled on an article recently noting that you can use the V8 pecl extension for running computationally-heavy code as JS from within PHP, rather than PHP code itself, to shave off some cycles.

A tool like this, with enough support, could allow you to easily convert large chunks of slower, PHP code (like object graph hydration) into JS, run in V8 for speed, and use the result in PHP without manually converting libraries.

One could have a chunk of php code that needs to be run client side (for whatever reason, offline access on a chromebook being one).

PHP.js could come in handy (no need for porting the whole thing to js). Especially if this code will change over time.

You're spot on. People asking 'why' or 'how is this useful?' are missing the point.

you should waste your time solving more useful puzzles. Head into Machine Learning for example, or if you are into VMs, why not try creating a Java VM on top of Cocoa for Android developers to invade iOS, that'll have more impact and you will have to learn some serious compiler theory.