将XML 2.0与PHP 5.6结合使用

我正在尝试解析我附加的下一个xml。问题是我无法用PHP5.6解析下一行。我想提取属性

<vuln:vulnerable-configuration id="http://www.nist.gov/">
  <cpe-lang:logical-test operator="OR" negate="false">
    <cpe-lang:fact-ref name="cpe:/o:microsoft:windows_8:-"/>
    <cpe-lang:fact-ref name="cpe:/o:microsoft:windows_8.1:-"/>
    <cpe-lang:fact-ref name="cpe:/o:microsoft:windows_server_2012:-:gold"/>
    <cpe-lang:fact-ref name="cpe:/o:microsoft:windows_rt:-:gold"/>
    <cpe-lang:fact-ref name="cpe:/o:microsoft:windows_rt_8.1:-"/>
  </cpe-lang:logical-test>
</vuln:vulnerable-configuration>

如果我打印变量$tmp，则表示数组为空。 当我打印$config时，它会显示：

Warning: main(): Node no longer exists in C:\xampp\htdocs\prueba\probando.php on line 56
Notice: Trying to get property of non-object in C:\xampp\htdocs\prueba\probando.php on line 56
Array ( [fact-ref] => )

谢谢

Warning: main(): Node no longer exists in C:\xampp\htdocs\prueba\probando.php on line 56
Notice: Trying to get property of non-object in C:\xampp\htdocs\prueba\probando.php on line 56
Array ( [fact-ref] => )