code: public function login()
      {
          $this->site_title = '欢迎使用';
  
          $login_url = Url::build('start/doLogin');
          $this->assign('login_url', $login_url);
  
          $code_url = Captcha::getCodeSrc(self::LOGIN_KEY);
          $this->assign('code_url', $code_url);
  
          return $this->fetch();
      }
docs: 登录页面