public string SendQuery(string query)
        {
            UTF8Encoding enc = new UTF8Encoding();
            byte[] postdata = enc.GetBytes("query=" + query);

            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(SerUrl);
            webRequest.CookieContainer = cookies;
            webRequest.Method = "POST";
            webRequest.ContentType = "application/x-www-form-urlencoded";
            webRequest.ContentLength = postdata.Length;
            webRequest.Headers.Add(HttpRequestHeader.AcceptCharset, "UTF-8");

            Stream stream = webRequest.GetRequestStream();
            stream.Write(postdata, 0, postdata.Length);
            stream.Close();

            HttpWebResponse response = (HttpWebResponse)webRequest.GetResponse();
            stream = response.GetResponseStream();

            StreamReader reader = new StreamReader(stream, Encoding.UTF8);
            string ReturnData = reader.ReadToEnd();

            return ReturnData;
        }

PHP má výchozí kódování UTF-8. Když odešlu třeba "+ěščřžýá" a na php hned nechám poslat zpět - echo $_POST["query"] - tak mi to vypíše +???????;