Using a filter similar to watchdog module

I am building a module and have copied the same filter code used in the watchdog module. It works great as is, but I want it so when a particular select list is changed, it will reload the page and alter the options of another select list. All is working, apart from in the function similar to 'watchdog_form_overview' which output the filter form - I want to access one of my $_SESSION variables to use in a query to change the options of the select list. The $_SESSION variables are not what is submitted as the function 'watchdog_form_overview_submit' which changes the $_SESSION variables is called after the filter form is outputed. I was wondering if there is a way to work around this or if there is another way to do a simple filter form.

Thanks.