How to use if clauses in ON condition of a MySQL-Join?

I want to user 2 IF clauses in an ON condition of a MySQL join. This is my not working code:

$db->joins = 'JOIN st_users as users ON
                IF(messages.toUserID = 0, messages.userID = users.ID,
                    IF(messages.userID = 2, messages.toUserID = users.ID, messages.userID = users.ID)
                    )';

Why its not working?