Setup HTTP expires headers using PHP and Apache

This Apache module might be of help: http://httpd.apache.org/docs/2.0/mod/mod_expires.html

There are two ways to do this. The first is to specify the header in your php code. This is great if you want to programatically adjust the expiry time. For example a wiki could set a longer expires time for a page which is not edited very often.

header('Expires: '.gmdate('D, d M Y H:i:s \G\M\T', time() + (60 * 60))); // 1 hour

Your second choice is to create an .htaccess file or modify your httpd config. In a shared hosting environment, modifying your .htaccess file is quite common. In order to do this, you need to know if your server supports mod_expires, mod_headers or both. The easiest way is simply trial and error, but some Apache servers are configured to let you view this information via the /server-info page. If your server has both mod_expires and mod_headers, and you want to set the expiry on static resources, try putting this in your .htaccess file:

# Turn on Expires and set default to 0
ExpiresActive On
ExpiresDefault A0

# Set up caching on media files for 1 year (forever?)
<FilesMatch "\.(flv|ico|pdf|avi|mov|ppt|doc|mp3|wmv|wav)$">
ExpiresDefault A29030400
Header append Cache-Control "public"
</FilesMatch>

For other combinations and more examples see: http://www.askapache.com/htaccess/speed-up-your-site-with-caching-and-cache-control.html

Did you try something like?

<?php
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
?>