it has been a while since my last post
I came across a problem with php session files on debian some time ago. On debian the default config of PHP disables session garbage collection. This is because the default session path is not writeable by the webserver.
The default session path is cleaned up by a cron job.
I wrote a blog post about this (only german):
ISPConfig sets up a different session path for the webs - that is a good solution - but:
If the PHP software (script) running doesn't set the garbage collection the session files are never deleted. On a highly visited page this can be a problem.
The problem is even bigger if you store your session data inside a database or even worse when storing inside a MEMORY table.
As mentioned in the blog post i set
in all my scripts to re-enable garbage collection.
I would recommend that ispconfig sets this variable for all websites in the vhosts file just for the case that the default setting is "disabled" as it is on debian.