Codeigniter注册控制器代码审查

我刚刚开始使用MVC框架，特别是Codeigniter，我在维护我的代码和在哪里放置我的功能(控制器或模型)方面遇到了一些麻烦。

现在我正在建立一个注册系统，我有一个控制器的名称注册。php

这是我的代码:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
Class Signup extends CI_Controller {
    public function __construct()
    {
        parent::__construct();
    }
    public function index()
    {
        $this->form_validation->set_rules('username', 'Username', 'trim|required|callback_check_valid_username|min_length[6]|max_length[20]|xss_clean');
        $this->form_validation->set_rules('email', 'Email', 'trim|required|valid_email');
        $this->form_validation->set_rules('password', 'Password', 'trim|required|min_length[6]|max_length[32]');
        if ($this->form_validation->run() == false){
            $this->load->view("register/index");
        }else{
            $this->submitRegistration();
        }
    }
    public function ajaxup(){
        if ($this->input->isAjaxRequest()){
            header('Content-type: application/json');
            $error = false;
            $message = '';
            $this->form_validation->set_rules('username', 'Username', 'trim|required|callback_check_valid_username|min_length[6]|max_length[20]|xss_clean');
            $this->form_validation->set_rules('email', 'Email', 'trim|required|valid_email');
            $this->form_validation->set_rules('password', 'Password', 'trim|required|min_length[6]|max_length[32]');
            if ($this->form_validation->run() == false){
                $message = validation_errors();
                $error = true;
            }else{
                $this->_submitRegistration();
                $message = 'Successfully registered.';
            }
            $return = array(
                'error' => $error,
                'message' => $message
            );
            $return = json_encode($return);
            echo $return;
        }
    }
    public function _submitRegistration(){
        $username = $this->input->post('username');
        $email = $this->input->post('email');
        $password = $this->input->post('password');
        $data = array(
            'username' => $username,
            'email'    => $email,
            'password' => $password
        );
        $this->load->model('users_model');
        $this->users_model->register_user($data);
    }
    public function check_valid_username($username){
        $this->load->model('users_model');
        if (!$this->users_model->is_valid_username($username)){
            $this->form_validation->set_message('check_valid_username', 'The %s field should contain only letters, numbers or periods');
            return false;
        }
        return true;
    }
}

有什么我可以写得更好，以维护我的代码和可读性?

*注意:*函数ajaxup在用户点击按钮并进行ajax调用时使用。

谢谢