spent some hours upgrading from mw v29 to v31 . without any changes to LocalSettings.php LDAP Authentication to debian 9 OpenLDAP server (slapd) works.
after reviewing my incomplete notes and apt install logs, the only thing I see we did was:
apt install php-ldap
our existing LocalSettings.php had:
require_once 'extensions/LdapAuthentication/LdapAuthentication.php';
$wgAuth = new LdapAuthenticationPlugin();