Php 未找到Amp类

我是Ampp的新手，我想先试试这个非常简单的代码。 我下载了AmHP with composer for windows，并保存了在我的项目文件夹中创建的所有文件夹

composer require amphp/http-client

那么我的代码是：

<?php

require __DIR__ . './autoload.php';

use Amp\Http\Client\HttpClientBuilder;
use Amp\Http\Client\HttpException;
use Amp\Http\Client\Request;
use Amp\Http\Client\Response;
use Amp\Loop;

$stringa = 'http://www.google.it/';

$request = new Request($stringa, "GET");
$location = $request->getHeader("Location");

var_dump($location);