This is what I have in my version (1.4.4) I don't know if you're running version 2 or not, but this will work anyway.

Code:
$url_admin = "$url_phpbb/admin";
$url_images = "$url_phpbb/images";
$url_smiles = "$url_images/smiles";
$url_phpbb_index = $url_phpbb . '/index.' . $phpEx;
$url_admin_index = $url_admin . '/index.' . $phpEx;