Unlike in the mysql world, Postgresql often doesn't listen on a network interface and connections occur via domain sockets. In PHP interfaces, this is typically specified by omitting any hostname parameter in the connection call.
Dokuwiki's pgsql auth module doesn't allow for this - leaving a blank server name results in pre-emptive failure.
The attached is my patch to allow for an empty $conf['auth']['pgsql']['server'], and use it as an indication to connect over domain sockets.