I've actually played around with this concept. Conceptually it's not very difficult (or at least it wasn't for the code I was working with)... The biggest issue is PHP's reliance on global functions vs Ruby's methods for strings, numbers, arrays, etc. So, for every PHP global string function, for instance, you'd need to pull out one of the arguments and call an equivalent method on it. Bleh.
(Did I just feel all of the HN rubyists shudder at once?)