Magento (the popular open-source online shop system) likes to store its PHP session files in ~/public_html/var/session/

Most debian servers don’t have that in their cron job that deletes old session files.

So, you probably want to set it to store it’s session files in the default location (/var/lib/php5) or alter your cron job (/etc/cron.d/php5)

Fun!