php解析XML在5.2版本环境报错，5.3版本通常

php解析XML在5.2版本环境报错，5.3版本正常。
if (isset($_POST['xmldata'])) {
$string="".$_POST['xmldata']."";
$doc=$xml = new DOMDocument();
$xml->loadXML($string);
...
为什么我表单提交过来的XML文件，页面报错呢？
页面提示信息：
Warning: DOMDocument::loadXML() [domdocument.loadxml]: AttValue: " or ' expected in Entity, line: 2 in /usr/local/apache2/htdocs/dbmanage/xmlfile.php on line 126

Warning: DOMDocument::loadXML() [domdocument.loadxml]: attributes construct error in Entity, line: 2 in /usr/local/apache2/htdocs/dbmanage/xmlfile.php on line 126

Warning: DOMDocument::loadXML() [domdocument.loadxml]: Couldn't find end of Start Tag add line 2 in Entity, line: 2 in /usr/local/apache2/htdocs/dbmanage/xmlfile.php on line 126

Warning: DOMDocument::loadXML() [domdocument.loadxml]: AttValue: " or ' expected in Entity, line: 3 in /usr/local/apache2/htdocs/dbmanage/xmlfile.php on line 126

Warning: DOMDocument::loadXML() [domdocument.loadxml]: attributes construct error in Entity, line: 3 in /usr/local/apache2/htdocs/dbmanage/xmlfile.php on line 126

xml格式如下：
 
 
 
 
 
 
 
 

PHP版本：
#php -v
PHP 5.2.12 (cli) (built: Apr 25 2012 21:12:31)

------解决方案--------------------
var_dump($string); // 加上这句，贴出结果来。
$xml->loadXML($string);


------解决方案--------------------
表单POST上来的？enctype是什么? 还是AJAX提上来的？ Content-Type设置了什么？ 看看HTTP_RAW_POST_DATA？