Can't get Discord's username with PHP, oauth2 and cURL [duplicate]

Question

My problem:

I would like to get the discord's username of a person when they have accepted it(with PHP). For that to happen, I created an HTML <a> that redirects users to the oauth2 Discord page when they have clicked on it. After they accepted, it returns me a code with a GET request.

And here is the problem. I have the code, but I didn't succeed in getting the person's username.

My code:

<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
ini_set('max_execution_time', 300); //300 seconds = 5 minutes. In case if your CURL is slow and is loading too much (Can be IPv6 problem)

error_reporting(E_ALL);

$tokenURL = 'https://discord.com/api/oauth2/token';

session_start();

// When Discord redirects the user back here, there will be a "code" and "state" parameter in the query string
if(get('code')) {
  echo get('code'); // Output the correct code
  // Exchange the auth code for a token
  $token = apiRequest($tokenURL, array(
    "grant_type" => "authorization_code",
    'client_id' =>'the_client_id',// It's not written 'the_client_id' but the real code
    'client_secret' => 'the_client_secret',// It's not written 'the_client_secret' but the real client secret code
    'code' => get('code')
  ));

  $_SESSION['access_token'] = $token->access_token;
}


function apiRequest($url, $post=FALSE, $headers=array()) {
  $ch = curl_init($url);
  curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);

  $response = curl_exec($ch);


  if($post)
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post));

  $headers[] = 'Accept: application/json';

  if(session('access_token'))
    $headers[] = 'Authorization: Bearer ' . session('access_token');

  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

  $response = curl_exec($ch);
  return json_decode($response);
}

function get($key, $default=NULL) { // Get the $key properly
  return array_key_exists($key, $_GET) ? $_GET[$key] : $default;
}

function session($key, $default=NULL) {
  return array_key_exists($key, $_SESSION) ? $_SESSION[$key] : $default;
}
?>

What is the problem that I have?

The problem is that the $token is null. So, I tried to see why is it null, and it's null because:

$response = curl_exec($ch); is false

So I checked in my php.ini and with phpinfo() that cURL was enabled, and it is. So I don't know where is the problem

I also checked that my client_secret code was correct by resetting it, so I don't think that it is the problem.

There is some similars questions like:

Discord oauth2 with php

Setting up a Discord oauth2 login on my website (with PHP?)

PHP Discord OAUTH2 code sample not working

But this is not the same problem. So don't put this question as [DUPLICATE] please

(I knew that was going to happend, but I reiterate what I said, it's not duplicated)