How to kill a/all php sessions?

Question

I have a very basic php session login script. I want to force logout of a certain user or force logout of all users.

How can I read all sessions made to my website,

Answer 1

Updated - Aug 2012

This code is based from the official PHP site, and another well written snippet on SO.

Works well. Servers like NGinx you can turn off, clean cache, swipe memory reset, clear logs etc and generally remove temp usage. Even drop the limits of memory.