wasteful retransmission of pages. However, if the content on your page potentially
changes with each request, it can be annoying if an old version appears. If you are
developing an e-commerce site, it can be critical that each page is processed anew.
On the other hand, your page may be dynamically building a page that contains
information that doesn't change very often. My experience has been that caches are smart
enough to store URLs that appear to be ordinary HTML files, but not URLs that contains
variables following a question mark. Your PHP may use variables in the URL, though. If
the information on these pages changes infrequently, you want to let the cache know.
RFC 2616 describes the HTTP 1.1 protocol, which offers several headers for controlling
the cache. Listing 18.3 shows the headers to send to prevent a page from being cached.
The Last-Modified header reports the last time a document was changed, and setting
it to the current time tells the browser this version of the page is fresh. The Expires
header tells the browser when this version of the document will become stale and should
be requested again. Again we use the current time, hopefully causing the browser to keep
the document out the cache. Perhaps the most important header, Cache-Control tells
the browser how to cache the page. In this situation, weare requesting the page not be
cached. The fourth header is for the benefit of older browsers that understand only HTTP
1.0. Try reloading thescript in Listing 18.3 rapidly. You should see the date update
each time.
Listing 18.3 Sending Headers to Prevent Caching
<?
header("Last-Modified: ". gmdate("D, d M Y
H:i:s"). " GMT");
header("Expires: ". gmdate("D, d M Y H:i:s"). "
GMT");
header("Cache-Control: no-cache, must-
revalidate");
header("Pragma: no-cache");
?>
The time is
Listing 18.4 causes a page to be cached for 24 hours. Like Listing 18.3, the Last-
Modified, Expires and Cache-Control headers are used to control cache
behavior. The last modification time is sent as the actual modification of the file. The
expiration time is sent as 24 hours from now. And the cache is instructed to let the
document age for 86,400 seconds, the number of seconds in a day. To prove to yourself