@CliveStHelmet Right now, it is essentially the same. But self references the same class where it is declared, and static references the class it's being used.

So, let's say later you implement another class that extends Session and rewrite the flush() method to do additional things:

<?php

class MyNewSession extends Session
{
    public static flush()
    {
        $_SESSION = [];

        // do something more
    }
}

Then, if you used self::flush() in the destroy() method in the parent class, when you call MyNewSession::destroy() it will use the flush() method from the Session class instead of the one in MyNewSession. Whereas if you used static::flush(), it will use the flush() method you rewrite in your MyNewSession class.

The PRG Pattern (and Session Flashing)