This is a small sample adapted from the xwebSecureClass that shows calling the Authenticate method and getting the soap headers.
NOTE: This code was adapted on the fly and is untested right now, if you find success or bugs please update the code accordingly.
$nfxweb = new SoapClient("https://mynf.mywork.org/xweb/Secure/netFORUMXML.asmx?WSDL",
Array('trace'=>true, //turning on trace=true will let us grab the headers and responses
//these are the params set in the constructor