Why does Debian clean php sessions with a cron job instead of using php's built-in garbage collector?

Debian and derivatives (Ubuntu) don't use the php session garbage collector

session.gc_probability = 0

instead they use a cron /etc/cron.d/php5

09,39 * * * * root [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete

Why Debian has chosen to do this?

Solution 1:

Because Debian sets very stringent permissions on /var/lib/php5 (1733, owner root, group root) to prevent PHP session hijacking. Unfortunately, this also prevents the native PHP session garbage collector from working, because it can't see the session files there. The cron job runs as root, which does have sufficient access to see and clean up the session files.

Edit: Supporting documentation: The behavior was established in response to bug #267720. (There used to be comments in the stock php.ini file about this, but I don't see them there now in my wheezy-based PHP install.)

Solution 2:

It's likely to be a bit more reliable on sites with low traffic (if you only get a couple hundred hits a day and GC only fires every thousand or so, sessions might stick around way longer than they should) and I imagine it might be a little less harsh on the server than the native GC if you've got lots of sessions.