if (isset($_POST['login_user'])) {
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $password = mysqli_real_escape_string($db, $_POST['password']);
  if (empty($username)) {
    array_push($errors, "Username is required");
  }
  if (empty($password)) {
    array_push($errors, "Password is required");
  }
  if (count($errors) == 0) {
    $password = md5($password);
    $query = "SELECT * FROM users WHERE username='$username' AND password='$password' AND email='$email'";
    $results = mysqli_query($db, $query);
    if (mysqli_num_rows($results) == 1) {
      $_SESSION['username'] = $username;
      $_SESSION['email'] = $email;
      $_SESSION['id'] = $id;
      $_SESSION['success'] = "You are now logged in";
      header('location: index.php');
    }else {
        array_push($errors, "Wrong username/password combination");
    }
  }
}

<div class="col s6">username</div>
        <div class="col s6"><?php echo $_SESSION['username']; ?></div>
        <div class="col s6">E-Mail</div>
        <div class="col s6"><?php echo $_SESSION['email']; ?></div>