PHP __SoapCall问题

Question

I have a simple service using php and Soap
There's a function named getPrice in my SOAP SERVER

This is getPrice :

    function getPrice($productISBN, $param)
    {

        return $productISBN;
    }

$kks = new SoapServer(null, array('uri' => "urn://my/webservice/shop"));

    $kks->addFunction( array(
                    'getPrice',
                    'reserveProduct',
                    'inquiryPostPrice',
                    'registerPostOrder'

                 ) );

    $kks->addFunction(SOAP_FUNCTIONS_ALL);

$kks->handle();

and this is my client file -This is in /kkc folder :

$client = new SoapClient(null, array(
      'location' => "http://example.com/webservice/kks/shop.php",
      'uri'      => "urn://my/webservice/shop"
    ) );

var_dump( $client->__soapCall('getPrice', array(123456789, $param) ) );

//$param is a string

but I'll get this error in client :

 Fatal error: Uncaught SoapFault exception: [HTTP] Internal Server
 Error in /example.com/webservice/kkc/client.php:28 Stack trace:
 #0 [internal function]: SoapClient->__doRequest('__soapCall('getPrice', Array)
 #2 {main}   thrown in /example.com/public_html/webservice/kkc/client.php on line 28

I have two other files like this and there's everything is fine...
Would you please let me know what's wrong here?
Thanks in Advance