This is a non-trivial issue and there is, sadly, no ready-made built-in-PHP method for this.
What you will have to do is to write your own session handler, driven by database. This has plenty of advantages, one of them being that PHP won’t garbage-collect your session data without you knowing about it. The other advantage is that you will be able to fine-tune how your sessions work.
This comes at a price, however, and the price is in raw code: writing a session handler, whilst being an awesome thing to do, can taketime.
Start by creating the following MySQL table:
sessions
id INTEGER NOT NULL, primary key, auto increment
sid VARCHAR(40), unique
last_active DATETIME
data TEXT
The idea is obvious: you cookie your users with a unique token and insert a session in the DB. The DB does stuff with it. How you store stuff is up to you.
The important part for you is what is called garbage collection. You can check which sessions are out of date using the last_active field in each session row. Every request on the page, check which ones are over, and when they are over, put the items back from the session to the store.
If you have a high-visit website, instead of doing it every request, do it every x requests so that you get one update per second or more at peak time.
