Adding a HTTP header to the Angular HttpClient doesn’t send the header, why?

Here is my code:

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.');
            }
        }
    );
}

and here the network debug:

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

and Data are stored in ‘Request Payload’ but in my server doesn’t received the POST values:

print_r($_POST);
Array
(
)

I believe the error comes from the header not set during the POST, what did I do wrong?