不要试图构建一个通用的转换。读取一个特定的对象结构并从它创建一个DOM要容易得多。这允许你定义结构，验证和重新格式化值，忽略不需要的值，...

$data = getData();

$document = new DOMDocument();
// create and append a document element
$document->appendChild(
    $stock = $document->createElement('stock')
);

// iterate data array
foreach ($data->EtatStock as $stockData) {
    // create and append an element for each item
    $stock->appendChild(
        $item = $document->createElement('item')
    );
    // add a child element with text content
    $item
      ->appendChild($document->createElement('CodeMag'))
      ->textContent = $stockData->CodeMag;
    $item
      ->appendChild($document->createElement('LibMag'))
      ->textContent = $stockData->LibMag;
    // add value as an attribute 
    $item->setAttribute('quantity', $stockData->QteStock);
}

$document->formatOutput = true;
echo $document->saveXML();

function getData() {
    $json = <<<'JSON'
{
    "EtatStock": [
        {
            "CodeMag": "001",
            "LibMag": "SO SHOCKING",
            "QteStock": 2000
        }
    ]
}    
JSON;
    return json_decode($json);
}

输出量：

<?xml version="1.0"?>
<stock>
  <item quantity="2000">
    <CodeMag>001</CodeMag>
    <LibMag>SO SHOCKING</LibMag>
  </item>
</stock>