YII基础准备
1.命名空间
<?php

/****假设有三个同名的类，输出的值为A，B,C****/

use a\b\c\apple;

use d\e\f\apple as bApple;

use g\h\i\apple;

$app = new apple();//A

$app = new bApple();//B

$app = new \Apple();//C  调用的是全局的

2.操作响应

<?php  
namespace app\controllers;

use yii\web\Controller;
use yii\data\Pagination;
use app\models\Country;

class CountryController extends Controller
{
    public function actionIndex()//不叫方法叫操作
    {
        $request = \YII::$app->request;//能够获取到url值
        echo $request->get('id',20);//如果没有传参可以设置默认值

        if($request->isGet) //isPut
        {
            echo "this is get method";
        }

        echo $request->userIP;//获取用户IP

        $res = \YII::$app->response;//获取响应状态
        $res->statusCode = 404;//人为设置响应状态码

        //$res->headers->add('pragma','no-cache');//设置head不设置缓存
        $res->headers->set('pragma','max-age=5');//设置head缓存5分？秒钟
        $res->headers->remove('pragma');

        //跳转
        $res->headers->add("location","http://www.baidu.com");
        $this->redirect("http://www.baidu.com",302);

        //文件下载
        $res->headers->add('content-disposition','attachment;filename="a.jpg"');
        $res->sendFile("robots.txt");

    }
}