php调用.net写的webservice，出现stdClass Object

按照下面的代码$client = new soapclient("http://xxxx/service.asmx?WSDL");
$param["Goodsno"]="XX"; 
$param["Inventoryno"]=""; 
$param["ColorName"]=""; 
$param["CmName"]=""; 
$param["BrandName"]=""; 
$res = $client->__Call('GetInventory',array('paramters'=>$param));
echo "<pre>";
print_r($res); 
echo "<pre>";
打印出来的数据是
stdClass Object
(
    [GetInventoryResult] => stdClass Object
        (
            [ArrayOfString] => Array
                (
                    [0] => stdClass Object
                        (
                              [string] => Array
                                (
                                    [0] => xx                                    
                                    [1] => xx                                    
                                    [2] => xx                                    
                                    [3] => xx                                    
                                    [4] => xx                               
                               )                        )
要怎么样才能解析出[0] => xx[1] => xx[2] => xx[3] => xx[4] => xx的值呢
我在$res = $client->__Call('GetInventory',array('paramters'=>$param));后面加->GetInventoryResult->string，返回的是NULL求助求助求助~~~~