php读取xml节点值,该如何处理

php读取xml节点值
获取的XML如下：

<?xml version="1.0" encoding="utf-8"?>
<string xmlns="SuperMarketService">{"status":"failed","result":"未知错误"}</string>

我如何获取string值呢

------解决思路----------------------

header('Content-type: text/html;charset=utf-8');
$s =<<< XML
<?xml version="1.0" encoding="utf-8"?>
<string xmlns="SuperMarketService">{"status":"failed","result":"未知错误"}</string>
XML;

$x = @simplexml_load_string($s);
echo $x[0];

{"status":"failed","result":"未知错误"}