How to talk to LDAP from an app?

How do I talk to an LDAP server from within an app’s controller? I’ve looked into the user_ldap app, but it does not come with instructions.

I tried calling the helper with
new \OCA\User_LDAP\Helper(\OC::$server->getConfig());
but that just tells me the class is not found.

Normally you should not need to directly talk to a specific user backend.
Mind explaining what your app does and what specific you are trying to do with the helper?