As horrible as PHP is, it is really easy to sit down with PHP and a tutorial or a book and start banging out code that gives you instant results. PHP is like the MS Basic 2.0 of the web era -- people who aren't really programmers (yet?) can sit down with it and GTD right away -- their code may be ugly but they can see it working as they go. They don't have to learn what a 'WAR' directory is and how it is structured, they don't have to mess around with xml configurations, etc. The ubiquity helps, certainly, but IMO the reason it got ubiquitous is how easy it is to go from nothing to "hello web world".
I think a project that maintains this instant GTD environment but improves the language is actually quite a good idea, though I also think cutting ties with PHP by giving it a new name makes more sense than calling it phpreboot and once you involve Java it seems less likely PHP's context-free easy start-up will be maintained.