在PHP框架中采用MVC模式时应避免哪些常见错误？

// index.php (控制器)
use Core\Request;
use Core\Session;

public function login() {
    $this->load->view('login_form', $data);
}

public function validate() {
    $request = new Request();
    $username = $request->getInput('email');
    $password = $request->getInput('password');

    if ($this->userModel->validate($username, $password)) {
        $session = new Session();
        $session->set('user_id', $this->userModel->getUserId());
        $this->changePage('dashboard');
    } else {
        $this->load->view('login_form', ['error' => 'Invalid credentials']);
    }
}

// loginModel.php (模型)
public function validate($username, $password) {
    $sql = "SELECT * FROM users WHERE email = ? AND password = ?";
    $stmt = $this->connection->prepare($sql);
    $stmt->bind_param('ss', $username, $password);
    $stmt->execute();
    $result = $stmt->get_result();
    return $result->num_rows > 0;
}

// login_form.php (视图)
<form action="/validate" method="post">
    <input type="email" name="email" placeholder="Email" />
    <input type="password" name="password" placeholder="Password" />
    <?php if (isset($error)) { ?>
        <span style="color:red;"><?php echo $error; ?></span>
    <?php } ?>
    <input type="submit" value="Login" />
</form>