Ah, you're right about PHP.. it has functions for working with shared memory. Didn't think of that to start with as the thing i'd previously done had to be a 100% self contained module
If you want to do it without Zend, then the PHP SHM functions are at
http://www.php.net/shmop
Edit: Remember that if your server dies, you're going to loose all of your data if it's only in memory. Might be worth at least writing it to a DB every so often as a backup.