Form validation: Callback not changing the input value

Code:

function _validate_birthday()

        {

            // Validates the birthdate from the user input.

            $day     = (int)$this->input->post('birthday');

            $month    = (int)$this->input->post('birthday_month');

            $year    = (int)$this->input->post('birthday_year');

            

            if (0 > $day || $day > 32) {

                $this->validation->set_message('validate_birthday', 'Invalid day in birthdate.');

                return false;

            }

            

            if (0 > $month || $month > 13) {

                $this->validation->set_message('validate_birthday', 'Invalid month in birthdate.');

                return false;

            }

            

            if (0 > $day || $day > (int)date('Y')) {

                $this->validation->set_message('validate_birthday', 'Invalid year in birthdate.');

                return false;

            }

            

            $birthday = mktime(0, 0, 0, $month, $day, $year);

            

            if ($birthday > strtotime('-18 years')) {

                $this->validation->set_message('validate_birthday', 'You must be 18 years old to sign up.');

                return false;

            }

            

            return $birthday;

        }

Code:

function signup()

        {

            $this->load->library(array('validation', 'email'));

            

            $rules['username']                = 'trim|required|min_length[5]|max_length[50]|callback__username_check|xss_clean|htmlentities';

            $rules['password']                = 'trim|required|matches[password_2]';

            $rules['password_2']            = 'trim|required';

            $rules['email']                     = 'trim|required|matches[email_2]|valid_email|callback__email_check';

            $rules['email_2']                = 'trim|required';

            $rules['first_name']            = 'trim|required|xss_clean';

            $rules['last_name']                = 'trim|required|xss_clean';

            $rules['state']                    = 'trim|required|exact_length[2]|alpha';

            $rules['street']                = 'trim|required';

            $rules['zip']                    = 'trim|required';

            $rules['birthday_month']        = 'trim|required';

            $rules['birthday_year']            = 'trim|required';

            $rules['birthday']                = 'trim|required|callback__validate_birthday';

            

            $this->validation->set_rules($rules);

            

            $fields['username']                = 'Username';

            $fields['password']                = 'Password';

            $fields['password_2']            = 'Repeated password';

            $fields['email']                 = 'Email';

            $fields['email_2']                = 'Repeated email';

            $fields['first_name']            = 'First name';

            $fields['middle_name']            = 'Middle name';

            $fields['last_name']            = 'Last name';

            $fields['phone']                = 'Phone';

            $fields['birthday_month']        = 'Birthmonth';

            $fields['birthday']                = 'Birthday';

            $fields['birthday_year']        = 'Birthyear';

            $fields['state']                = 'State';

            $fields['street']                = 'Street';

            $fields['street_2']                = 'Street 2';

            $fields['zip']                    = 'Zip';

            

            $this->validation->set_fields($fields);

            

            $data = array('title' => 'Sign up', 'where' => array('sale-rent' => false, 'buyer-renter' => false, 'forum' => false, 'blogs' => false));

            

            if ($this->validation->run() === false) {

                // Show the signup form

                // Either because it's the users first request

                // or because validation wasn't passed.

                $data['content'] = $this->load->view('user/signup', false, true);

            } else {

                die($this->input->post('birthday'));

                exit;

                // Success! Save the user, send an email, profit!

                $register = $this->redux_auth->register($this->input->post('username'), $this->input->post('password'), $this->input->post('email'));

                $data['content'] = $this->load->view('user/signup-success', false, true);

            }

                

            $this->load->view('main_template', $data);

        }