This way base_dn() method is executed multiple times, it should cache the result in memory to not call domain_info() every time. I suppose we could even check if the value contains replacement macro before we call expand().

See inline comments. Other than that it looks good. Still we could improve performance for cases where value does not contain base_dn expansion macro. I think it would be good to improve Conf::expand() method so $custom could accept values of type function, then we could probably (untested) do:

$base_dn_fn = function() use ($this) { return $this->base_dn(); };
$this->conf->expand($_value, $custom = array('base_dn' => $base_dn_fn));

Of course , we can do this later.

Updated to reflect comments.