Mturk : How do serialize DOM into XML using PHP..?

$QuestionXML = '<QuestionForm xmlns="../QuestionForm.xsd">
                <Question>
                   <QuestionContent>
                      <Text>

                         <![CDATA['.$thequestion.']]>

                      </Text>
                   </QuestionContent>
                <AnswerSpecification>
                   <FreeTextAnswer/>
                </AnswerSpecification>
                </Question>
               </QuestionForm>';