PHP SOAP pass form array to WSDL on remote server

Dear Experts.

I am stuck on a small problem with PHP5 Soap / WSDL.

I need to pass the contents of a form to fill a WSDL on another server. This needs to be sent as an array.

The array is sent as a $param and I have attemted to do it like this:

$weborder = array('OrderID'=>$OrderID,'UserName'=>$UserName,'Password'=>$Password,'CusID'=>$DealerID,'SiteID'=>$DealSiteID,'Title'=>$title,'CusFname'=>$fname,'CusLName'=>$lname, 'BillAddr1'=>$addr1,  'BillAddr2'=>$addr2, 'BillAddr3'=>$addr3, 'BillAddr4'=>$addr4, 'BillPCode'=>$postc, 'BillPhone'=>$phone, 'BillMobile'=>$mobile,  'BillEmail'=>$email);

$param["WebsiteOrder"] = $Weborder;


$client = new SoapClient("http://www.somewhere.com/service.asmx?WSDL");
$RIOresult = $client -> ReadInOrder($param);

And my hope was that it would populate this WSDL correctly

 <ReadInOrder xmlns="http://www.somewhere.com/">
    <OrderID>$OrderID</OrderID>
      <UserName>$UserName</UserName>
      <Password>$Password</Password>
      <WebsiteOrder>
        <CusID>$DealerID</CusID>
        <SiteID>$DealSiteID</SiteID>
        <Title>$title</Title>
        <CusFName>$fname</CusFName>
        <CusLName>$lname</CusLName>
        <BillAddr1>$addr1</BillAddr1>
        <BillAddr2>$addr2</BillAddr2>
        <BillAddr3>$addr3</BillAddr3>
        <BillAddr4>$addr4</BillAddr4>
        <BillPCode>$postc</BillPCode>
        <BillPhone>$phone</BillPhone>
        <BillMobile>$mobile</BillMobile>
        <BillEmail>$email</BillEmail>

 </WebsiteOrder>
    </ReadInOrder>

But the best I can get is this error

Uncaught SoapFault exception: [soap:Server] Server was unable to process request. ---> Object reference not set to an instance of an object. in C:\Inetpub\wwwroot........etc

Can anyone help me decifer the problem and set me on the path to enlightenment!

many thanx