Aleksey Martynov has submitted a project that can come in quite handy of you do a lot of work with XML in your application - a XML serialization tool for PHP 5.3 that lets you easily translate objects into valid XML.
lexa-xml-serialization is a lightweight library for serializing PHP objects to XML like it's done in .NET. My xml serializer requires that you mark serializable properties of your classes with doc-comment annotations. The properties are not required to be public. The lettercase of annotations doesn't matter. Whitespaces are not essential. Parameters are optional: type defaults to string, xmlName defaults to the property name. Parameter values must not be enclosed in quotes. Empty brackets may be omitted.
The page on the Google Code site gives some examples of it in use as well as as "first look" tutorial about building a large, more complex XML file. If you want to try it out, head over to the download page to grab the latest.