If you’re looking for a simple static solution (an not an entire ad manager) use this
add in your init.php after the $tpl = get_tpl(… line
if (0 == rand(0, 1)) {
$ad = ‘Google‘;
} else {
$ad = ‘Yahoo‘;
}
$tpl->assign(‘footer_ad’, $ad);
and in your footer use
{$footer_ad}
where you want your ad to show.