I canot call asp.net web servicr in php

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ASP.NET

PHP

webservice

I created web service using asp.net framework 4 :

[WebMethod] 
public DataSet GetMembers(string memberId, string thirdName) 
{ 
string connStr = ConfigurationManager.ConnectionStrings["conStr"].ConnectionString; 
SqlConnection cn = new SqlConnection(connStr); 
SqlDataAdapter da = new SqlDataAdapter(); 
DataSet ds = new DataSet();

da.SelectCommand = new SqlCommand(@"SELECT MemberMaster.MemberId,MemberMaster.FirstName,MemberMaster.SecondName,MemberMaster.ThirdName,MemberMaster.CivilNo,MemberMaster.DateOfBirth,MemberSubcription.MemberId
                                        FROM MemberMaster inner join MemberSubcription on MemberMaster.MemberId=MemberSubcription.MemberId
                                        WHERE MemberMaster.MemberId=@MemberId and MemberMaster.ThirdName=@ThirdName", cn);
    da.SelectCommand.Parameters.Add("@MemberId", SqlDbType.NVarChar).Value = memberId;
    da.SelectCommand.Parameters.Add("@ThirdName", SqlDbType.NVarChar).Value = thirdName;

    ds.Clear();
    da.Fill(ds);
    return ds;
}

it work good in localhost but when i call it in the php :

    <?php

$wsdlUrl = "http://xxxx/WS/WebService.asmx?WSDL";  

$client = new soapclient($wsdlUrl);

$params = array('memberId' => 'W1100045',
'thirdName' => 'Ali');              

$result = $client->GetMembers($params);

$echoText = '';

if (is_null($result->GetMembersResult))
{

echo "no result";
}

else
{
  echo $result->GetMembersResult;
}

?>

this error appear : Catchable fatal error: Object of class stdClass could not be converted to string on line 23

what is the problem and how can i solve this problem ?