How to Manage PHP Session Variables That Disappear When The User Is Inactive For a While

I set some PHP session variables when my user logs into my app.

Generally they work very well.

However if the user leaves their computer for a few hours, when they come back, the app is still running in the browser but the PHP session variables are no longer present, so the app randomly fails tasks that rely on them.

Not being a PHP guru, can anyone explain exactly what is going on here and the best way to manage this situation?

Thanks!
Antony.