php 正则提取XML 指定内容方法？求大拿帮忙（史上最难）

$string = <<<XML
<?xml version="1.0" encoding="utf-8"?>
<stream><MSGCODE>AAAAAAA</MSGCODE><MSGCN>交易成功！</MSGCN><B2CCERTNO>500415</B2CCERTNO><ORDERDATE>2013-11-21</ORDERDATE><ORDERTIME>09:52:23</ORDERTIME><ORDERNO>DGM20131121000009</ORDERNO><ORDERAMT>6.26</ORDERAMT><CURRID>01</CURRID><PAYAMT>6.26</PAYAMT><PAYNO>BC131121095428004271</PAYNO><ACCNO>6226********6770</ACCNO><ACCTYPE>01</ACCTYPE><PBCSTNAME>测试</PBCSTNAME><ACCHASH>fSSczJM0fims1Xpmrvi2pnBtrr4=</ACCHASH></stream>
XML;preg_match_all('#<(MSGCODE|ORDERNO)>(.+)</\1#s',$string,$m);
print_r($m);