向 Angular HttpClient 添加 HTTP 标头不会发送标头，为什么

以下是我的代码:

import { HttpClient, HttpErrorResponse, HttpHeaders } from '@angular/common/http';

logIn(username: string, password: string) {
    const url = 'http://server.com/index.php';
    const body = JSON.stringify({username: username,
                                 password: password});
    const headers = new HttpHeaders();
    headers.set('Content-Type', 'application/json; charset=utf-8');
    this.http.post(url, body, {headers: headers}).subscribe(
        (data) => {
            console.log(data);
        },
        (err: HttpErrorResponse) => {
            if (err.error instanceof Error) {
                console.log('Client-side error occured.');
            } else {
                console.log('Server-side error occured.');
            }
        }
    );
}

这里是网络调试:

Request Method:POST
Status Code:200 OK
Accept:application/json, text/plain, */*
Accept-Encoding:gzip, deflate
Accept-Language:en-US,en;q=0.8
Cache-Control:no-cache
Connection:keep-alive
Content-Length:46
Content-Type:text/plain

数据存储在"请求有效负载"中，但在我的服务器中没有收到POST值:

print_r($_POST);
Array
(
)

我相信这个错误来自帖子中没有设置的标题，我做错了什么？