cache control with javascript

Does anybody know how I can make force a javascript file (written in PHP) to be cached if the user agent allows it? Here is the situation: I am creating a dropdown menu system that contains a customer list, loaded from a database. This list is written to the javascript file for the menu. The menu can be quite large as the data grows. What I would like to do, is force it to be cached (unless of course the user agent doesn't allow it) so that it doesn't have to make the call to generate the file each time. I have searched through the php.net website and even through the HTTP/1.1 protocols. I am continuing to look, but have not found a definite answer as to whether or not what I am trying to do is possible. -- Joshua Minnie Lead Web Application Developer Advantage Computer Services, LLC www.advantagecomputerservices.com josh@acsurf.net