Call to undefined method soapclient::GetOrdersByClientPO()?

I am trying to consume a soap service in php. This is the code I am using

define('APIURL','https://cgorders.com/v2.1/Service.asmx?WSDL');
$client         =   new SoapClient(APIURL);
        $search_query   =   new StdClass();

        $search_query ->CustomerID      =   CustomerID;
        $search_query ->ClientPO        =   $orderID;
        $search_query ->AccessToken     =   $cgTokem;   //AccessToken;
        $result     =   $client->GetOrdersByClientPO($search_query);

        //echo  "<pre>";print_r($result->GetOrdersByClientPOResult->Orders->OrderID);echo   "</pre>";exit;
        if(isset($result->GetOrdersByClientPOResult->Orders->OrderID))
        {
            return($result->GetOrdersByClientPOResult->Orders->OrderID);
        }
        else
        {
            return('');
        }

I am passing appropriate parameters correctly, did not mention them, for security reasons. I am getting Call to undefined method soapclient::GetOrdersByClientPO() . Can anybody help?

Answer

Soap client was not installed over the server, I was testing. After installation, it worked like a charm. Thank you.