This is a bare framework. The "nice" bits haven't been put in yet. Specifically, no HTTP response checking is performed, and no type validation or coercion has been put in. PHP being a loosely-typed language, this is going to have to be done explicitly.
dateTime.iso8601 is supported opaquely. It can't be done natively as the XML-RPC specification explictly forbids passing of timezone specifiers in ISO8601 format dates. You can, however, use the iso8601_encode() and iso8601_decode() functions to do the encoding and decoding for you.
If alternative character set encoding is sent in HTTP header than it will be ignored for the moment. We speak only UTF-8...
If more than 32k of HTTP headers are encountered (like, why?) then the response parsing code will break.