Update UserFaker.class.php

UserFaker sets all users' lastActivityTime to 0. Meaning that those users never were logged in, although they might have been members for years. That is unrealistic and prevents you from seriously testing activity-related issues without creating session data or activities (posting, likes ...) for some of those users.

This modification sets lastActivityTime to random values between registrationDate and TIME_NOW or to 0, which results in a more reasonable mix of lastActivityTime values.