我尝试在登录后显示用户的名字，但它没有出现在页面上。我尝试这样显示它

<h2> Welcome <?php echo $this->session->userdata('firstname'); ?> </h2>

但是它没有显示用户的名字，尽管DB中有数据.但是当我尝试将'firstname'更改为'username'时，它显示了用户的电子邮件（test@gmail.com）.我如何显示用户的名字？下面是我的控制器：

function login(){
    $this->form_validation->set_rules('username', 'Username', 'required|valid_email');
    $this->form_validation->set_rules('password', 'Password', 'required');

    if ($this->form_validation->run() == FALSE) {
      redirect('/');

    } else {
      $user_login = array(
        'username' => $this->input->post('username'),
        'password' => $this->input->post('password'),
      );
      $login = new UI_model;
      $result = $login->login_user($user_login);

      if($result){
        $auth_details = array(
          'firstname' => $result->firstname,
          'lastname' => $result->lastname,
          'username' => $result->username,
        );

        $this->session->set_userdata($auth_details);
        // $this->session->set_userdata('authenticated', '1');
        $this->session->set_flashdata('status', 'User Logged in Successfully!');
        redirect(base_url('pages/index'));

      } else {
        $this->session->set_flashdata('status', 'Invalid Credentials. Please try again');
        redirect(base_url('pages/about'));
      }
    }
  }

以下是我的模型：

public function login_user($data){
        $this->db->select('*');
        $this->db->where('username', $data['username']);
        $this->db->where('password', $data['password']);
        $this->db->from('users');
        $this->db->limit(1);
        $query = $this->db->get();
        if($query->num_rows() == 1){
            return $query->row();
        } else {
            return false;
        }
    }