i would do this a completely different way.
I would use three tables to create a many to many relation of files to user groups:
files:
fileID - primary key
fileName - varchar
usergroups:
userGrouplID - Primary Key
userGroup - varchar
usergroupfiles:
usergroupfileID - primary key
usergroupfileFileID - relates to files.fileID
usergroupfileGroupID - relates to usergroups.userGrouplID
then you can use a join query to get all files that belong to a specific user group:
SELECT files.fileName
FROM files
INNER JOIN usergroupfiles
ON usergroupfiles.usergroupfileGroupID = files.fileID
WHERE usergroupfiles.usergroupfileGroupID = paramID
variables:
name: paramID
type: integer
default value = -1
run time value: $_SESSION[user_level]