My mail management system (currently codenamed NVMS) is in the backend managing userdb, and the web interface is currently written in PHP.
XML-RPC seemed the sensible thing to use, as it is simple and portable, Python and
Twisted has excellent and easy-to-use support, and I knew PHP could do it. (Well, not without getting sidetracked to determine which XML-RPC library to use for PHP.) I settled on PEAR's
XML_RPC. It's nowhere near as braindeadly simple and nice as Python's xmlrpclib (and Twisted's XML-RPC server support), so here's something I used to make PEAR's XML_RPC slightly easier to work with, and an example of how it works on the other side with Twisted.