I got my problem solved ;)
So, on the top of my login.php file, which is outside of the forum directory (phpBB3/) and on my normal website, I put the following code:
<?php
define('IN_PHPBB', true);
define('PHPBB_ROOT_PATH', './phpBB3/');
$phpbb_root_path = 'phpBB3/';
$phpEx =...