如何利用PHP的MVC架构构建灵活可维护的应用程序？

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(255) NOT NULL,
    password VARCHAR(255) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

<?php

class UserModel {
    public function createUser($username, $password) {
        // 将用户数据插入到数据库
    }
    
    public function getUser($username, $password) {
        // 从数据库中获取用户数据
    }
}

<?php

class RegisterView {
    public function display() {
        // 显示用户注册页面的HTML代码
    }
    
    public function showError($error) {
        // 显示错误信息的HTML代码
    }
}

<?php

class RegisterController {
    private $model;
    private $view;
    
    public function __construct($model, $view) {
        $this->model = $model;
        $this->view = $view;
    }
    
    public function register() {
        if ($_SERVER['REQUEST_METHOD'] === 'POST') {
            // 处理用户注册表单的提交
            $username = $_POST['username'];
            $password = $_POST['password'];
            
            $this->model->createUser($username, $password);
            
            // 注册成功，显示成功页面
        } else {
            // 显示用户注册页面
            $this->view->display();
        }
    }
}

<?php

require_once 'UserModel.php';
require_once 'RegisterView.php';
require_once 'RegisterController.php';

$model = new UserModel();
$view = new RegisterView();
$controller = new RegisterController($model, $view);

$controller->register();