修改xml文件值的有关问题

修改xml文件值的问题
我想改变question为22222的值，这个值是在前台传入的，不过我不知道应该如何传，应该如何操作了？最好是用DOMDocument类来操作

<?xml version="1.0" encoding="utf-8"?>
<parent>
  <title>3</title>
  <author>admin</author>
  <contentall>
    <subject>
      <question>111111</question>
      <option>
        <oid>A</oid>
        <content>11</content>
      </option>
      <option>
        <oid>B</oid>
        <content>22</content>
      </option>
      <option>
        <oid>C</oid>
        <content>33</content>
      </option>
      <option>
        <oid>D</oid>
        <content>44</content>
      </option>
    </subject>
    <subject>
      <question>22222</question>
      <option>
        <oid>A</oid>
        <content>22</content>
      </option>
      <option>
        <oid>B</oid>
        <content>11</content>
      </option>
      <option>
        <oid>C</oid>
        <content>44</content>
      </option>
      <option>
        <oid>D</oid>
        <content>33</content>
      </option>
    </subject>
    <subject>
      <question>33333</question>
      <option>
        <oid>A</oid>
        <content>444</content>
      </option>
      <option>
        <oid>B</oid>
        <content>222</content>
      </option>
      <option>
        <oid>C</oid>
        <content>333</content>
      </option>
      <option>
        <oid>D</oid>
        <content>2111</content> 
 
      </option>
    </subject>
  </contentall>
</parent>

$fn = '你的xml文件名';
$tag = 'question'; //待修改的节点名
$val = '22222'; //待修改的节点值，和 $tag 一起用于定位
$replace = 'xxx'; //修改后的值
$xml = simplexml_load_file($fn);
$p = $xml->xpath("//{$tag}[.='$val']");
$p[0][0] = $replace;
$xml->asXML($fn); //写回去