php simplexml怎么修改数据

<?php
/*  设置编码格式  */
header(&#39;Content-Type:text/html;charset=utf-8&#39;);
/*  创建XML格式的字符串  */
$str = <<<XML
<?xml version=&#39;1.0&#39; encoding=&#39;gb2312&#39;?>
<object name=&#39;商品&#39;>
	<book>
		<computerbook type=&#39;PHP入门应用&#39;>PHP从入门到精通</computerbook>
</book>
</object>
XML;
/*  ***************************  */
/*  创建SimpleXML对象  */
$xml = simplexml_load_string($str);
/*  输出根目录属性name的值  */
echo $xml[name].&#39;<br />&#39;;
/*  修改子元素computerbook的属性值type  */
$xml->book->computerbook[&#39;type&#39;] = iconv(&#39;gb2312&#39;,&#39;utf-8&#39;,&#39;PHP程序员必备工具&#39;);
/*  修改子元素computerbook的值  */
$xml->book->computerbook = iconv(&#39;gb2312&#39;,&#39;utf-8&#39;,&#39;PHP函数参考大全&#39;);
/*  输出修改后的属性和元素值  */
echo $xml->book->computerbook[&#39;type&#39;].&#39; => &#39;;
echo $xml->book->computerbook;
?>