My project is in zend framework and I want to increase the user inactivity time to 1-2 weeks. Cookies for my browser setting correctly but the session logs out user after 8 hour as I have set the session.gc_maxlifetime value to 28800. So I just wanted to confirm before moving forward that "Would it increase the server load, if I increase PHP session.gc_maxlifetime to 1-2 week?"
By default, session data is stored as serialized object in plain text files on the hard disk of your server. Higher session timeout then means more session files in the folder. You won't experience any significant increase in server load, but depending on the amount of sessions you might hit filesystem limitations (scan time increases with the amount of files in a folder)
Alternative session storage like MySQL might be a solution
Sessions are maintained on server for each user. Increase in session time out will prevent the Server from releasing memory allocated to inactive session.
So yes, if you have too many users and you keeping their session for week, then you will performance issue.