如何修改php simplexml中的元素？

我需要用用户提供的值更新xml节点；但我似乎无法修改simplexml对象。有什么想法吗

xml结构：

<xm>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
</xm>

---

这样做：

$units = @simplexml_load_file("../test.xml");
$i = 1;
foreach($units->unit as $unit)
{
    if($i == $_POST['row_id'])
    {
        $unit->building = $_POST['building_txt'];
    }
    $i++;
}

echo $units->asXML();

以下工作（您需要参考

单元

的结构，以指示要检索的行：

$xml = simplexml_load_string('<xm>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
</xm>');

$row = $_POST['row_id'] - 1;
$xml->unit[$row]->building = 'Test';

var_dump($xml->asXML());

$xml=simplexml\u load\u字符串（'
');
$row=$\u POST['row\u id']-1；
$xml->unit[$row]->building='Test'；
var_dump（$xml->asXML（））；

$xml = simplexml_load_string('<xm>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
   <unit>
        <building></building>
   </unit>
</xm>');

$row = $_POST['row_id'] - 1;
$xml->unit[$row]->building = 'Test';

var_dump($xml->asXML());