Before using please install php5-ldap module
sudo apt-get install php5-ldap

For using php ldap on windows please refer
http://stackoverflow.com/questions/16864306/fatal-error-call-to-undefined-function-ldap-connect


The user "admin" will always login with local db username and password (even LDAP is enabled)

Use following config to test LDAP connection with following test LDAP server
http://www.forumsys.com/tutorials/integration-how-to/ldap/online-ldap-test-server/

Change configs as follows under System->Settings

LDAP: Enabled = Yes
LDAP: Server = ldap.forumsys.com
LDAP: Port = 389
LDAP: Root DN = dc=example,dc=com
LDAP: Manager DN = cn=read-only-admin,dc=example,dc=com
LDAP: Manager Password = password
LDAP: Version 3 = Yes
LDAP: User Filter = uid={}

Then create a user with username "riemann" under System->Users

Logout and try login with riemann/password