the rich set of libraries and add-ons that are the real only argument for PHP.
The other argument being that you can run PHP on just about any shared host out there.
Plus, there's the fact that the built-in Arc application server assumes a persistent, threaded server process is running throughout the lifespan of your application. PHP scripts are a one-shot deal...
Nobody said it was easy, but is it possible?
I guess what it comes down to is that anyone using a shared host isn't really "serious" enough to be writing an app in Arc to begin with.
Of course it's possible. The question is whether it's a worthwhile use of valuable hacking time.
I also don't suspect it's a particularly interesting problem for the people most likely to be able to tackle it effectively. Now, a native Arc compiler for x86/AMD64, that would be a nice widget to have.