Website side:
$dB->query("INSERT INTO Me_MuOnline.dbo.forum_tmp (forum_fid,forum_nick) VALUES ('".$_SESSION['username']."','".$userid."')");
extract($_POST);


                if (isset($_SERVER["HTTP_CF_CONNECTING_IP"])) {
                 $_SERVER['REMOTE_ADDR'] = $_SERVER["HTTP_CF_CONNECTING_IP"];
                }

                $ch = curl_init();
                curl_setopt($ch, CURLOPT_URL,"http://forumurl/process.php");
                curl_setopt($ch, CURLOPT_POST, 1);
                $titkoskod='e2KULYiT0Zy~KT5l3J280cWO\6I5jh7s6NRHdYwTnH59po6aVz3GG0LEm2n4I5[trKEqJ$xX2yldw2%xQiQnnZhFV01]*zYy2Hxq';
                curl_setopt($ch, CURLOPT_POSTFIELDS,"username={$userid}&user_password={$memb__pwd}&user_email={$mail_addr}&ip={$_SERVER['REMOTE_ADDR']}&titkoskod={$secret}");
                curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
                $server_output = curl_exec ($ch);
                curl_close ($ch);

                header('Location: https://forumurl/');
                exit();
Process.php:

<?php
session_start();
ini_set("display_errors","on");
if($_POST["secret"]!=base64_decode('ZTJLVUxZaVQwWnl+S1Q1bDNKMjgwY1dPXDZJNWpoN3M2TlJIZFl3VG5INTlwbzZhVnozR0cwTEVtMm40STVbdHJLRXFKJHhYMnlsZHcyJXhRaVFublpoRlYwMV0qell5Mkh4cQ==')){ header('location: /'); exit; }
define('IN_PHPBB', true);
$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : '';

$phpEx = substr(strrchr(__FILE__, '.'), 1);
include($phpbb_root_path . 'common.' . $phpEx);
include($phpbb_root_path . 'includes/functions_display.' . $phpEx);
include($phpbb_root_path . 'includes/functions_user.' . $phpEx);

// Start session management
$user->session_begin();
$auth->acl($user->data);
$user->setup('');
$request->enable_super_globals();
$username=$_POST['username'];
$user_password=$_POST['user_password'];
$user_email=$_POST['user_email'];
$ip=$_POST['ip'];


$user_row = array(
    'username'              => $username,
    'user_password'         => phpbb_hash($user_password),
    'user_email'            => $user_email,
    'group_id'              => 2,
    'user_lang'             => 'hu',
    'user_type'             => USER_NORMAL,
    'user_ip'               => $ip,
    'user_regdate'          => time(),
);

//register and handle error
$user_id = user_add($user_row);
if ($user_id === false){
    //handle error
}
?>


Aaaaand you are done ;)