Problem with getting value from SoapResponse

Hi,

I've the problem that the SOAP Extension does not seem to parse the
response correctly. A small example:
I call a Webservice using a WSDL:
$objClient = new SoapClient(
                                  "mywsdl.wsdl"
                                 ,array(
                                         'trace'=>1
                                        ,'encoding'=>'ISO-8859-1'
                                        ,'location'=>'http://SOAPURL'
                                       )
                                );
and I call a function supplying the required parameters:
$objResult = $objClient->execute(
                                array(
                                       'context'=> $this->arrContext
                                      ,'input'=> $arrInput
                                      ,'output'=> $arrOutput
                                     )
                               );

The XML-SOAP-Response contains a paramter:
<SESSION_TOKEN xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:nil="false">819118e2f0f2f1f6f2e1</SESSION_TOKEN>

Problem: The resulting Soap-Object does NOT contain the parameter
value - the SESSION_TOKEN member is just NULL.

The previous version of the webservice returned the response in a different way:
<p828:SESSION_TOKEN>819118e2f0f2f1f6f2e1</p828:SESSION_TOKEN>
This kind of answer is parsed correctly by PHP - the Soap-Object
containts the SESSION_TOKEN member with the corresponding value.

Why is the value not parsed from the new response? Looks for me like a
PHP bug, as the response looks like a valid structure. Any hints?