php读取xml的有关问题

php读取xml的问题
我有一个这样的XML文件，我想把question、oid、content都输出，而且输出的格式为：
问题：542
选项：A 内容：1111111
选项：B 内容：2222222
问题：123
选项：C 内容：3333333
选项：D 内容：4444444
应该如何做？

<?xml version="1.0" encoding="utf-8"?><br /><parent><br />  <title>3</title><br />  <author>admin</author><br />  <contentall><br />    <subject><br />      <question>542</question><br />      <option><br />        <oid>A</oid><br />        <content>1111111</content><br />      </option><br />      <option><br />        <oid>B</oid><br />        <content>2222222</content><br />      </option><br />    </subject><br />    <subject><br />      <question>123</question><br />      <option><br />        <oid>C</oid><br />        <content>3333333</content><br />      </option><br />      <option><br />        <oid>D</oid><br />        <content>4444444</content><br />      </option><br />    </subject><br />  </contentall><br /></parent><br />

xml php
------解决方案--------------------

$s =<?xml  version="1.0" encoding="utf-8"?><br><parent><br>  <title>3</title><br>  <author>admin</author><br>  <contentall><br>    <subject><br>      <question>542</question><br>      <option>
<br>        <oid>A</oid><br>        <content>1111111</content><br>      </option>
<br>      <option>
<br>        <oid>B</oid><br>        <content>2222222</content><br>      </option>
<br>    </subject><br>    <subject><br>      <question>123</question><br>      <option>
<br>        <oid>C</oid><br>        <content>3333333</content><br>      </option>
<br>      <option>
<br>        <oid>D</oid><br>        <content>4444444</content><br>      </option>
<br>    </subject><br>  </contentall><br></parent><br>XML;<br><br>$xml = simplexml_load_string($s);<div class="clear">
                 
              
              
        
            </div>