Почему получаю ошибку: Cannot modify header information - headers already sent? [дубликат]

Ошибки:

Warning: Cannot modify header information - headers already sent by (output started at T:\home\MadSport.com\www\login.php:3) in T:\home\MadSport.com\www\login.php on line 13

Warning: Cannot modify header information - headers already sent by (output started at T:\home\MadSport.com\www\login.php:3) in T:\home\MadSport.com\www\login.php on line 15

Warning: Cannot modify header information - headers already sent by (output started at T:\home\MadSport.com\www\login.php:3) in T:\home\MadSport.com\www\login.php on line 16

Код:

<?php
 $dbc = mysqli_connect('localhost', 'root', '', 'lesson');
if(!isset($_COOKIE['user_id'])) {
  if(isset($_POST['submit'])) {
    $user_username = mysqli_real_escape_string($dbc, trim($_POST['username']));
    $user_password = mysqli_real_escape_string($dbc, trim($_POST['password']));
    if(!empty($user_username) && !empty($user_password)) {
      $query = "SELECT `user_id` , `username` FROM `signup` WHERE username = '$user_username' AND password = SHA('$user_password')";
      $data = mysqli_query($dbc,$query);
      if(mysqli_num_rows($data) == 1) {
        header('Location: index.php');
        $row = mysqli_fetch_assoc($data);
        setcookie('user_id', $row['user_id'], time() + (60*60*24*30));
        setcookie('username', $row['username'], time() + (60*60*24*30)); 
      }
      else {
        echo 'Sorry,you should correctly write password!';
      }
    } else {
      echo 'Correct the fields!';
    }
  }
}
?>