Post data to PHP

I have sample code for sending data from asp to PHP, but something goes wrong.
Here is the code
           
            StringBuilder postData = new StringBuilder(); string url="http://1.1.1.1/api.php";

                          postData.Append("security=" + HttpUtility.UrlEncode(Md5AddSecret(txtGuid.Text)) + "&");
              postData.Append("auth=" + HttpUtility.UrlEncode("dsadasdsaads") + "&");
              postData.Append("action=&qu ...