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>