HttpWebRequest httpWReq =
    (HttpWebRequest)WebRequest.Create("http://www.site.com/mht.php");

ASCIIEncoding encoding = new ASCIIEncoding();
string postData = "mail=xxx@hotmail.com&sifre=12345";
byte[] data = encoding.GetBytes(postData);

httpWReq.Method = "POST";
httpWReq.ContentType = "application/x-www-form-urlencoded";
httpWReq.ContentLength = data.Length;

using (Stream stream = httpWReq.GetRequestStream())
{
    stream.Write(data,0,data.Length);
}

HttpWebResponse response = (HttpWebResponse)httpWReq.GetResponse();

string responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();
    MessageBox.Show("İşlem Sonucu: " responseString);