V konfiguračním souboru: php.ini

Někde tady:

; Define the probability that the 'garbage collection' process is started
; on every session initialization.
; The probability is calculated by using gc_probability/gc_divisor,
; e.g. 1/100 means there is a 1% chance that the GC process starts
; on each request.

; This is disabled in the Debian packages, due to the strict permissions
; on /var/lib/php4. Instead of setting this here, see the cronjob at
; /etc/cron.d/php4, which uses the session.gc_maxlifetime setting below
;session.gc_probability = 0
session.gc_divisor = 100

; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
session.gc_maxlifetime = 1440