Php/xml/DOMDocument

xml取得

$dom = new DOMDocument();
$dom->loadXML($xml);
$x = $dom->documentElement;
$key = $x->getElementsByTagName("key");
return $key->item(0)->nodeValue;

<?xml version="1.0" encoding="utf-8" ?>
<info xml:lang="ja">
  <key>test</key>
</info>

参考

http://php.net/manual/ja/domelement.getattribute.php