CodeIgniter custom validate function not working

Have a look at the following documentation: http://ellislab.com/codeigniter/user-guide/libraries/form_validation.html#callbacks

As you can see int the documentation, the custom validation function actually belongs in the controller, and not in the config file. By moving the validation function to the controller, the callback function should start getting called.

Another fun fact, people can access this unique_email function through a url (ie. http://yoursite.com/index.php/user/unique_email). To avoid this, we can write the function as a private function by simply placing an underscore at the beginning of the function, like so:

function _unique_email($email) {
    ...
}

You can then call the function in your validation by using the new function name in your config (notice the extra underscore in the callback:

array(
    'field' => 'email',
    'label' => 'Email',
    'rules' => 'required|valid_email|callback__unique_email'
)

In the end, your controller should look similar to the following:

class User extends CI_Controller {

    public function register() {
        $this->load->helper('form');
        $data['message'] = '';
        if($this->input->post('submit')) {
            $this->load->library('form_validation');
            if($this->form_validation->run() == FALSE) {
                $data['message'] = 'User could not be saved.';
            } else {
                $user_data['firstname'] = $this->input->post('firstname');
                $user_data['lastname'] = $this->input->post('lastname');
                $user_data['email'] = $this->input->post('email');
                $user_data['password'] = md5($this->input->post('password'));
                if($this->user_model->insert($user_data)) {
                    if($this->user_model->login($user_data)) {
                        $this->session->set_flashdata('message', 'User saved successfully.');
                        redirect('/user', 'refresh');
                    }
                }
            }
        } 

        $this->load->view('user/register', $data);
    }

    function _unique_email($email) {
        if($email == '[email protected]') {
            $this->form_validation->set_message('unique_email', 'Hello World !');
            return false;
        }
    }

}

Your config would look similar to the following:

$config = array(
    array(
        'field' => 'firstname',
        'label' => 'First Name',
        'rules' => 'required'
    ),
    array(
        'field' => 'lastname',
        'label' => 'Last Name',
        'rules' => 'required'
    ),
    array(
        'field' => 'email',
        'label' => 'Email',
        'rules' => 'required|valid_email|callback__unique_email'
    ),
    array(
        'field' => 'password',
        'label' => 'Password',
        'rules' => 'required|matches[confirm_password]'
    ),
    array(
        'field' => 'confirm_password',
        'label' => 'Confirm Password',
        'rules' => 'required'
    )
);