PHP XML Serializer

PEAR contains an XML_Serializer and XML_Unserializer class. These classes allow you to convert an aribritrary PHP datastructure to XML and back. As PHP is dynamically typed this XML has to contain type (classes) information.

SitePoint:

These days, XML has become part of landscape in most all areas of software development — none more so than on the Web. Those using common XML applications, such as RSS and XML-RPC, will probably find public domain libraries geared specifically to help them work with the formats, eliminating the need for wheel re-invention.