> Now, it should not take anything more than a 
> few days to build a SOAP server, and make PHP send a SOAP 
> message to this server where the returned message contains 
> the transformed XML.

Why SOAP? It can be done perfectly well using REST.

There are also tools available such as

http://php-java-bridge.sourceforge.net/pjb/

I haven't used it, though one of my clients is planning an evaluation.

Michael Kay
http://www.saxonica.com/