Cannot exit in CodeIgniter constructor

CodeIgniter does not instantly output the view when you load it with $this->load->view();, it puts the output of the view to the output buffer instead and after everything is loaded it flushes the whole output to the user. With that said, the problem you are seeing is that it buffers the output, then it dies without flushing it.

die is really bad and should not be used outside debugging, you should better use something like a variable switch. If it's only for the controllers scope, then you can make a

private $show_everything_else = true;

In the constructor:

if( $this->username == "" || empty( $this->username)){
    $this->load->view( 'access/login');
    $this->show_everything_else = false;
}

In any of the controller's methods:

if($this->show_everything_else) {
    // ...
}

In any case, this solution is a quick fix and there are much better possibilities how to do this, but all of them require a different approach.

You can have a method and call it in constructor:

function __construct() {
    parent::__construct();
    $this->_is_logged_in();
}

and the method should look like this:

function _is_logged_in() {
    $is_logged_in = $this->session->userdata('is_logged_in');
    if (!isset($is_logged_in) || $is_logged_in != true) {
        redirect('login');
        die();
    }
}

And, of course, you should have controller for login, which can look like this:

<?php

class Login extends CI_Controller {

    function index() {
        $this->load->view('LOGIN-VIEW');
    }

    function validate() {
        $this->load->model('login_model');
        $data = $this->login_model->validate();

        if ($data != false) {
            $data['is_logged_in'] = true;
            $this->session->set_userdata($data);
            redirect('MAIN-CONTROLLER-AFTER-LOGIN');
        }
        else {
            $this->index();
        }
    }

    function logout() {
        $this->session->sess_destroy();
        $this->index();
    }

}

This what i posted, also preserve sessions in database.

Login model can be as primitive as this:

class Login_model extends CI_Model {

function validate() {


    $this->db->where('username', $this->input->post('username'));
    $this->db->where('password', md5($this->input->post('password')));
    $query = $this->db->get('users');

    if($query->num_rows == 1) {
        $data = $query->row_array();
             return $data;
        } else {
            return false;
        }
}

The Controller class will automatically run the index() function. So, to make it work, you must return the index() if the login check in construct is failed. Here what you can do:

private $username = null;
private $mongoid = null;
private $neoid = null;
private $no_login = false;

public function __construct(){
    parent::__construct();

    // session to global
    $this->username = $this->session->userdata( 'username');
    $this->mongoid = $this->session->userdata( 'mongoid');
    $this->neoid = $this->session->userdata( 'neoid');

    // check if user is logged in
    if( $this->username == "" || empty( $this->username)){
        $this->load->view( 'access/login');
        $this->no_login = true;
    }
}

function index(){
    if($this->no_login) return;
    // another statement if login success.....
}