求助：Fatal error: Uncaught Error: Class 'http\Client' not found in D:\phpstudy_pro

php报错： Fatal error: Uncaught Error: Class 'http\Client' not found in D:\phpstudy_pro\WWW....

<?php
$client = new http\Client;
$request = new http\Client\Request;
$request->setRequestUrl('https://dev-api.');
$request->setRequestMethod('POST');
$body = new http\Message\Body;

执行到第2行就出错了，是不是没有内置的类？请问怎么解决？