When dealing with large amounts of data, try to use caching instead of going to database for every request.

PHP is a scripting language, which basically means there is an "exe" somewhere reading the php script every time there is a request. For php that will be Zend Engine.

Assuming $inactiv is an integer:

if (!isset($inactiv)) $inactiv = 0;
else $inactiv = my_time() - intval($inactiv) * 2592000;

It will be better if you make sure the $inactiv is always a number