I got the solutions,thanks for your help.
create a new WebClient object
WebClient client = new WebClient();
string url = "http://testurl.com/test.php";
create a byte array for holding the returned data
byte[] html = client.DownloadData(url);
use the UTF8Encoding object to convert the byte array into a string
UTF8Encoding utf = new UTF8Encoding();
get the converted string
string mystring = utf.GetString(html);