login.js

$('form').on('submit', function(e) {
        e.preventDefault();
        $.ajax({
            type: "POST",
            url: $(this).attr('action'),
            data: $(this).serialize(),
            success: function(resp) {
                var obj = $.parseJSON(resp);
                if (obj.resp == 0) {
                    $(this).alertBox(obj.err, { type: 'error'});
                } else {

                }
            }
        });
        return false;
    });
    
    
    <?php

class user extends BM_Controller {

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

    function login() {
        if (isset($_POST['username'])) {
            $this->load->model('bm_users');
            $user = $this->bm_users->login($_POST);
            if ($user !== 'failed') {
                $data['1'];
            } else {
                $data['resp'] = '0';
                $data['err'] = 'The username/password combination did not match. Please try again.';
            }

            $dta = json_encode($data);
            echo $dta;
        }
        $this->load->view('login');
    }

}