PHP App User Permissions Tecnique

I am building a web based community forum type software using PHP. I am currently working on how different user privelages are handled. Each user is part of a group and each group has varying degrees of privelages. I need to add in functionality which at the begginning of every script check the permission level of the user accessing the page. There are about 15 different permissions. I am debating whether i should store all the permission abilities into session variables and they can quickly be checked throughout the script, or whether i should query the database to check to see if the user has adequate group permissions to perform a certain function. Could someone give me some advice as to which would be more advisable?