Post dữ liệu về webserver với HttpURLConnection

http://ntdan.byethost6.com/android/chungthuc.php 

// mo ket noi de goi du lieu len server web
URL url = new URL(address);
HttpURLConnection con = (HttpURLConnection) url.openConnection();

// theo thu tu cua lenh execute(user,pass);
String user = params[0];
String pass = params[1];

String urlParams = "username=" + URLEncoder.encode(user, "UTF-8") +
 "&password=" + URLEncoder.encode(pass, "UTF-8");

con.setRequestMethod("POST");
con.setDoOutput(true);

DataOutputStream dos = new DataOutputStream(con.getOutputStream());
// chuyern user va pass cho web server
dos.writeBytes(urlParams);
dos.flush();
dos.close();

InputStream in = con.getInputStream();

StringBuilder str = new StringBuilder();
String line = "";
BufferedReader br = new BufferedReader(new InputStreamReader(in));
while ((line = br.readLine()) != null) {
 str.append(line);
 str.append("\n");
}
br.close();

Mã nguồn PHP

if(isset($_POST['username']) && isset($_POST['password']))
{
        mysql_connect("localhost","root","") or die(mysql_error());
        mysql_select_db("android");
        $username = isset($_POST['username']) ? $_POST['username'] : "";
       //$_POST['username'];
        $password = isset($_POST['password']) ? $_POST['password'] : "";
       //$_POST['password'];
        $sqlSelect = "select username,hinh from nguoidung where username='$username'";
        $users = mysql_query($sqlSelect);
 
  if(mysql_num_rows($users) > 0)
  {
       echo json_encode(mysql_fetch_assoc($users)); 
       $_SESSION['user'] = "ok";
  }else{
        echo json_encode(array("loi"=>"user khong hop le"));
  }else
  { 
        echo json_encode(array("loi"=>"user khong hop le"));
  }
}