Content Type issue in http requests (still text/html when I change it to application/json)

Question

I try to write a service in PHP and my code does not work.

This is my code

<?php
    $data_array = array();

    $data_file_contents = file_get_contents("../data/credentials.txt");
    $u_p = explode(" ", $data_file_contents);
    
    $host=$u_p[0];
    $uname=$u_p[1];
    $pass=$u_p[2];
    $dbname=$u_p[3];
    
    $dbh = mysqli_connect($host,$uname,$pass) or die("cannot connect");
    mysqli_select_db($dbh, $dbname);
    
    mysqli_close($dbh);

    $data_array['host'] = $host;
    $data_array['username'] = $uname;
    $data_array['password'] = $pass;
    $data_array['dbname'] = $dbname;
    $data =  json_encode($data_array);

    header("Content-Type: application/json");
    echo $data;
    exit();
?>

The output is correct and in json format as I want but the response in an HTTP request in reqbin for example is this:

Server: nginx
Date: Wed, 18 May 2022 19:56:16 GMT
Content-Type: text/html
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept-Encoding
Expires: Thu, 01 Jan 1970 00:00:01 GMT
Cache-Control: no-cache
Content-Encoding: gzip

This is some java code to see if that works but it doesn't

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;


public class http{
    public static void main(String[] args) throws IOException{
        URL url = new URL("http://myAddress/service.php");
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod("GET");
        con.setRequestProperty("Accept", "application/json");
        String contentType = con.getContentType();
        System.out.println("Content Type: " + contentType);
        con.disconnect();
    }
}