Polskie forum wsparcia dla phpBB

You'll have to put the sitemap.php file at the domain's root level, since no link from above
the sitemap files folder are taken into consideration by Google.
To do this, you only need to open the file you want to move up, and look for :

$phpbb_root_path = './';

and change it to :

$phpbb_root_path = './phpbb/';

Obviously replacing phpbb/ with the real folder name where phpBB would be installed, in sitemap.php.
You can do the same with rss.php and urllist.txt if you wish

// Correct syntax : "./phpbb/"
$phpbb_root_path = './home/bhp/public_html/forum/';