Well that just makes me go WTF. I genuinely can't think of a way this will be used which will be positive for PHP.
In theory, it will have the exact same use as the rubyracer gem ( http://rubygems.org/gems/therubyracer ). Possible uses of that are compiling coffee script using the official compiler or running the same form validation code on both the client- and the server side. In general I'd be cautious about both the ruby racer and this as you are practically embedding a garbage-collected heap into another garbage collected he…
That sounds great until you realize that you usually do a lot more validation on the server than on the client (is the username taken, is password correct, etc etc). So you would still have to use PHP for validation (in most cases).