[Xenon]

you have to add something like that in your index.php:

$DB_site->query("UPDATE user SET ppd=posts/((UNIX_TIMESTAMP(NOW())-joindate)/86400)");

basically i wouldn't do that into index.php, because it is run so often.
put it into memberlist.php, i think there you show up the users sorted by ppd right?

second: sorry i don't understand what you want, can you explain it a bit more? (sorry, perhaps its my english )