Static Pages

Normally you wont have to do anything with static pages, just make sure your server support the IF_MODIFIED_SINCE header.

To check whether or not your server supports the If-Modified-Since Header, simply upload a simple HTML page to your server, request it with your browser, edit it, and request it again. If the content in your browser changed, then its safe to assume that everything is working.

A more secure way to check if its working, is to download a plugin for your browser, which shows the headers, and response codes. Firefox has a number of such useful plugins.

Dynamic Pages

This includes websites made with server-sided scripting, such as PHP, and/or programming. These pages normally don't send the correct headers, unless they are coded, or programmed to do so.

If-Modified-Since with PHP

Below is an example of how to use IF_MODIFIED_SINCE with PHP.

$fp = fopen($_SERVER["SCRIPT_FILENAME"], "r"); 
$etag = md5(serialize(fstat($fp))); 
fclose($fp); 
header('Etag: '.$etag); 
header("Last-Modified: ".gmdate("D, d M Y H:i:s", $SelectS['LTIME'])." GMT");
header("Etag: $etag"); 

You should note that the $SelectS['LTIME'] Variable holds the timestamp from the LTIME field in your database, this is the date/time at which the requested content was last modified. The timestamp is saved in the database, using the unix format with php time(), but it may be better to use a real date format, to avoid having to deal with leap years etc.

Note. Its a good idea to have a field in your table, for both the date of postage, and the date of which the entry was last edited.

Now its just a PHP if then else, situation. I.e.

if ((@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $SelectS['LTIME']) && ( 
    trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag)) { 
    header("HTTP/1.1 304 Not Modified");
    exit;
}

$fp = fopen($_SERVER["SCRIPT_FILENAME"], "r"); 
$etag = md5(serialize(fstat($fp))); 
fclose($fp); 
header('Etag: '.$etag); 
header("Last-Modified: ".gmdate("D, d M Y H:i:s", $SelectS['LTIME'])." GMT");
header("Etag: $etag"); 

if ((@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $SelectS['LTIME']) && ( 
    trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag)) { 
    header("HTTP/1.1 304 Not Modified");
    exit;
}

Benefits

The main benefits are related to bandwidth usage. Sites that make use of the IF MODIFIED SINCE Header, generally save a lot of bandwidth.