By changing the values in the form, you can generate a document with specific Expires and Last-Modified headers. The RFC field refers to the format used to represent time. RFC1123 is preferred but some programs generate RFC850. Agents and servers should understand both formats.
Apache will cache the URLs generated by this form; however, Squid cache
will not by default (you must comment out "http_stop ?" in
squid.conf). Alternatively, you can encode the arguments to the script
in PATH_INFO instead of QUERY_STRING, like this:
Expires tonight, modified last night
No expiry, modified last night
No expiry, no last-modified
Expires: 0, modified last night
Expires in 1 minute, modified last night
More on Proxy Cacheing Servers
Cache in
Netscape
what is a cache
(Netscape)