1. 单入口,public/index.php
2. 实例化应用app, 以下$this均为app实例
new __construct, from \yii\base\Application
app继承体系中父类的构造,from:Object
yii\base\Application::init(), 启动引导程序
3. app->run(), from (父类)yii\base\Application
web-app->handleRequest, 引导路由(routing)
yii\web\response->send, 向客户端发送HTTP请求结果
over, return exitStatus