Código PHP:
<?php
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);
// Start session management
$user->session_begin();
$auth->acl($user->data);
$user->setup();
page_header('Title Here');
if ($user->data['user_id'] == ANONYMOUS)
{
login_box('', $user->lang['LOGIN']);
} else{
$id = "archivo.zip";
$enlace = "Directorio/".$id;
header ("Content-Disposition: attachment; filename=".$id." ");
header ("Content-Type: application/zip");
header ("Content-Length: ".filesize($enlace));
readfile($enlace);
}
make_jumpbox(append_sid("{$phpbb_root_path}viewforum.$phpEx"));
page_footer();
?>
Intenta con eso