Expiring other resources with HTTP headers

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Hello everybody,

I write here to find out if this is possible:

I want to expire an static file in the web browser through an HTTP
header (Expires, Cache-Control or something else) sent from a PHP
program.  The usual thing is that those headers apply only to the
program or file sending those headers, what I want is that a program
sends those headers to affect a different file in the browser cache.

In more detail, My web app generates a JavaScript file from the
information stored in the database, that file represents the user menu
and it is what the user sees in the web browser.  Now, through an apache
directive I set expiration times for several kinds of static resources
so the web server doesn't get slash dotted with lots of unnecessary
requests (this is a web app with lots of traffic).

Although the JavaScript files are generated from PHP, it is seen as
static content from the web browser POV.  Right now I have set the
expiration time for the menu files to 20 minutes.  But let's say an
admin user change the permissions of a user and the affected user hits a
link that says "Regenerate menu".  What will happen is that the
JavaScript file will be regenerated on the web server but there is a
chance that the browser won't see the change because the file is still
valid in the cache.

I know I could reduce the expiration time to reduce this problem but
most of the time those files do not change.  What can I do to notify the
web browser that the file in the cache is no longer valid?

I hope there is enough information here and that somebody can give me a
hint in the right direction.

Thanks,

-William

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