使用所有XMLNS解析SOAP信封 - 不在Magento上工作

I have tried and tried and have been unable to come up with a solution. My issue is this: I have a SOAP envelop response which is as follows...

    <soapenv:envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
    <soapenv:header>
        <soapenv:body>
            <mcu:prescreenusereligibilityresponse xmlns:mcu="http://www.ups.com/XMLSchema/XOLTWS/MCUserEligibility/v1.0">
                <common:response xmlns:common="http://www.ups.com/XMLSchema/XOLTWS/Common/v1.0">
                    <common:responsestatus>
                        <common:code>1</common:code>
                        <common:description>Success</common:description>
                    </common:responsestatus>
                </common:response>
                <mcu:eligibilitystatuscode>3</mcu:eligibilitystatuscode>
            </mcu:prescreenusereligibilityresponse>
        </soapenv:body>
    </soapenv:header>
</soapenv:envelope>

I then access the elements like this on my mac:

$ns=array();
$xml=new SimpleXMLElement($string);
foreach($xml->getNamespaces(true) as $key=>$url){
    $xml->registerXPathNamespace($key, $url);
    $ns[]=strval($url);

}
print_r(strval($xml->children($ns[0])->header->body->children($ns[1])->prescreenusereligibilityresponse->eligibilitystatuscode));

Using the same method on a separate Linux instance I get an error on the print_r line, saying the last child cannot be null. I have confirmed that the values are correct. I have also tried using $xml->xpath('//mcu:eligibilitystatuscode') with no success.

I'm really stuck -_-