XML无法解析&如何解决

XML无法解析&，怎么解决啊

PHP code

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$str1 = "http://user.qzone.qq.com/1340477530#!app=2&pos=1300412602";
$xml = '<?xml version="1.0" encoding="utf-8"?>'."\r\n";
$xml .= '<user>'."\r\n";
$xml .= "\t".'<person_siteurl>'.$str1.'</person_siteurl>'."\r\n";
$xml .= '</user>'."\r\n";
$sxe = new SimpleXMLElement($xml);
$sxe->asXML('test.xml');

运行后发现如下错误，

HTML code

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arning: SimpleXMLElement::__construct() [simplexmlelement.--construct]: Entity: line 3: parser error : EntityRef: expecting ';' in C:\wamp\www\test\a.php on line 18
Warning: SimpleXMLElement::__construct() [simplexmlelement.--construct]: <person_siteurl>http://user.qzone.qq.com/1340477530#!app=2\&pos=1300412602in C:\wamp\www\test\a.php on line 18

</person_siteurl>

将$str1中的 "&" 去掉后就正常了，这是为什么啊。。有什么解决办法吗？谢谢~~


------解决方案--------------------
$xml .= "\t".''.htmlentities($str1).''."\r\n";