php obj -> xml ?

Just wanted some opinions on the best way to produce well-formed xml from a php object. In my case, I was toying with the idea of a generic way to product an xml file from a PEAR DataObject - the idea being that a 3rd party requests data, I extract from mysql using Pear's DataObjects, then tell that objects to produce xml (or pass it to a transformer or something). Links? Ideas? Recommendations from RL experience? cheers, neko