怎么我用PHP调用.net 的webservice参数传不过去。参数为空

代码是：
PHP:
$client = new SoapClient('http://webservice.jxdcw.com/Service.asmx?wsdl',true); 
 
 
$client->soap_defencoding   =   'UTF-8';
$aryPara = array('UserName'=>'sdfsdfsdfsd');
$res= $client->call('Set_Cookie',$aryPara); 
.net
    [WebMethod(EnableSession = true)]
    public string Set_Cookie(string UserName)
    {
        if (UserName == "" || UserName == null)
        {
            return "132132132";
        }
        else
        {
            if (System.Web.HttpContext.Current.Request.Cookies["PhP_User"] == null)
            {
                HttpCookie Co = new HttpCookie("PhP_User", UserName);
                Co.Expires = DateTime.Now.AddDays(3);
                System.Web.HttpContext.Current.Response.Cookies.Add(Co);
                return Co.Value;            }
            else
            {
                return "已经创建";
            }
        }
    }
高手帮下忙。我就差这一步了