how to call a soap webservice with struct input parameters?

I want to interact with SOAP (as a client) and am not able to get the right syntax for input parameters. I have a WSDL URL that I have tested it with SoapUI and it retus result properly. There are two functions defined in the WSDL, but I only need one ("FirstFunction" below). Here is the script I run to get information on the available functions and types:

$client = new SoapClient("http://example.com/webservices?wsdl");
var_dump($client->__getFunctions()); 
var_dump($client->__getTypes()); 

And here is the output it generates:

array(
  [0] => "FirstFunction Function1(FirstFunction $parameters)",
  [1] => "SecondFunction Function2(SecondFunction $parameters)",
);

struct Amount {
 anyURI Identifier;
 Information charge;
 string referenceCode;
}

struct Information {
 string description;
 decimal amount;
 string code;
}

According to above result I developed my client with nusoap and php as below:

 class Information 
  {
    public $description;
    public $amount;
    public $code;
  }

  class Amount {

        public $Identifier;
        public $charge;
        public $referenceCode;
  }
$charge = new Information();
$charge->description = "ROUTE=XXX|abc=".$code;
$charge->amount = "NULL";
$charge->code = $chargecode;

$params = new chargeAmount();
$params->Identifier =$num;
$params->charge = $charge;
$params->referenceCode = $refcode;

$header = new SoapHeader('key', $key);

$client->__setSoapHeaders($header);


try
    {
        $res = $client->__call('charge',array('parametrs'=>$params)); 

        print_r($res->retu);
    }
catch(PDOException $e)
    {
    print_r($e->getMessage());
    }

I get the following error as result:

Uncaught SoapFault exception: [soapenv:Server] unknown