Yet another interesting situation is when you are using a ready-made php application, that provides support for the XMLRPC protocol via the native php xmlrpc extension, but the extension is not available on your php install (e.g. because of shared hosting constraints).
Since version 2.1, the PHP-XMLRPC library provides a compatibility layer that aims to be 100% compliant with the xmlrpc extension API. This means that any code written to run on the extension should obtain the exact same results, albeit using more resources and a longer processing time, using the PHP-XMLRPC library and the extension compatibility module. The module is part of the EXTRAS package, available as a separate download from the sourceforge.net website, since version 0.2