Remember atime will not be updated by simple read access to the file unless you take care for that in the mounting parameters of your filesystem.
Instead of atime, mtime will be delivered. But mtime only will be updated, if the contents of the file has changed.
Due to this behavior take care in your Session-System. Using session_start() will not be sufficiant to keep your sessions alive. You should write into the Session, e. g. :
$_SESSION = time();
That retriggers your mtime and atime, even if the mounting parameter for atime is set to "noatime" or is missing, dependent of the used file System.