Session theory

At one time, I thought that I read about a problem with PHP4 sessions involving the recognitioni of the session expiration in the session engine. The problem was that the session variables were read into the scipt BEFORE the expiration time was checked. Anyone remember this? So, I'm asking if there's anyone here with an AUTHORITATIVE answer I ( i.e. you've tested this, or you've written on the code for PHP sessions) to which order the following three task run when a script is requested? ------------------------------------------------------------ [A] read session file and load session vars if file and PHPSESSID both exist at request time [B] delete sesssion if expired [C] run garbage collection