getting the value of a javascript variable

hi everybody, i have a short problem: i have two drop down menues in a form. the content of the second dropdown is dependent from the selection in the first dropdown menue. now i want write a short javascript which runs on the onSelect - event of the first dropdown. this script gives me the selected entry of the first dropdown menue. now i have to give this value somehow into the PHP-environment for a DB-Query. The feched data i want dynamically fill into the second dropdown. But how do I get the value from the javascript variable into the php environment without an submit of the form?? has enybody a solution for this? thanx so far - hermann...