Codeigniter – My form validation callback doesn’t work

I have tried to copy from Codegniter’s documentation, but I can’t make form validation callbacks working.

I added helper form, url and library form_validation. It’s not working and always returns “false”

Controller

 public function addtest()
        {
            $this->load->helper(array('form', 'url'));

            $this->load->library('form_validation');

            $this->form_validation->set_rules('username', 'Username', 'callback_username_check');
            if($_SERVER['REQUEST_METHOD'] == 'POST')
            {
                 if ($this->form_validation->run() == TRUE)
                {
                       die('true');
                }
                else
                {
                       die('false');
                }
            }

            $this->template
                ->build('myform',array());
        }

    public function username_check($str)
    {
        if ($str == 'test')
    {
            return TRUE;
    }
    else
    {
            return FALSE;
    }
    }

View

<form method="post" action="" class="form-horizontal form-label-left">
    <div class="col-xs-12 col-md-9">
        <div class="x_panel">
            <div class="form-group col-xs-12">
                <div class="col-xs-3">
                    <label class="control-label">Folder name</label>
                </div>
                <div class="col-xs-9">
                    <input type="text" name="username" value="" class="form-control " id="" placeholder="">
                </div>
            </div>
            <button type="submit" class="btn btn-success">Submit</button>
        </div>
    </div>
</form>

Answer

Extend your form_validation library in Libraries.php

MY_Form_validation.php

<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class MY_Form_validation extends CI_Form_validation
{
    function run($module = '', $group = '') {

        (is_object($module)) AND $this->CI =& $module;
        return parent::run($group);
    }
}
/* End of file MY_Form_validation.php */
/* Location: ./application/libraries/MY_Form_validation.php */