Using Google, getting answers to such questions often is a matter of seconds. The reply by eric256 contained several suitable keywords to craft a search, e.g. http://www.google.com/search?q=authzldapauthoritative (choose a word that looks sufficiently specific to the issue...). The first hit is a link to the Apache auth module mod_authnz_ldap on the httpd.apache.org site, which has lots of other info around configuring Apache.