How should I continue this code - php

Question

I am trying to obtain the username variable from an external PHP script that is not part of Drupal or a Drupal Module.
For Drupal 7 there was drupal_bootstrap to load the required Drupal dependencies. However, after a long search all I found for Drupal 8 was a note that drupal_bootstrap is deprecated for Drupal 8.
And indeed, I get a
Call to undefined function drupal_bootstrap()
when I try to call it after including as in
<?php
define('DRUPAL_ROOT', '/var/www/html/drupal7'); // Define the DRUPAL_ROOT constant which is used in many places.
$base_url = 'http://'.$_SERVER['HTTP_HOST'].'/drupal7'; // Again, this is used in many places.
require_once DRUPAL_ROOT . '/includes/bootstrap.inc'; // Include the boostrap file.
drupal_bootstrap(DRUPAL_BOOTSTRAP_SESSION); // Start things up to whatever level you need.
global $user;
echo $user->name;
So how can I use the Drupal 8 API from an outside script?!
Posted this on another forum and I got this code, but it just doesn't work, so I would like to know what's going wrong. I know that something's missing, but I don't know how to echo the name of the current user who's logged on:
<?php
define('DRUPAL_DIR', __DIR__ .'/../../drupal');
use Drupal\Core\DrupalKernel;
use Symfony\Component\HttpFoundation\Request;
$autoloader = require_once DRUPAL_DIR . '/autoload.php';
$request = Request::createFromGlobals();
$kernel = DrupalKernel::createFromRequest($request, $autoloader, 'prod');
$kernel->boot();
require_once DRUPAL_DIR . '/core/includes/database.inc';
require_once DRUPAL_DIR . '/core/includes/schema.inc';
?>
Thanks in advance