$xml = '<?xml version="1.0" encoding="utf-8"?>
<APIResult xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://tempuri.org/">
  <Code>0</Code>
  <Result>短信提交成功</Result>
</APIResult>';

$xml = iconv('gbk', 'utf-8', $xml);//转了也不行

$dom = simplexml_load_string($xml);

$result_code = iconv('utf-8', 'gbk', $dom->code);//结果代码
$result_info = iconv('utf-8', 'gbk', $dom->result);//结果信息