Using this approach, you need to send a Last-Modified header every time your PHP script is accessed. The next time the browser requests the page, it sends an If-Modified-Since header containing a time; your script can then identify whether the page has been updated since the time provided. If it hasn’t, your script sends an HTTP 304 status code to indicate that the page has not been modified, and exits before sending the body of the page.

Here’s the code:

<?php
// Include PEAR::Cache_Lite
require_once 'Cache/Lite.php';

// Define options for Cache_Lite
$options = array(
  'cacheDir' => './cache/'
);

// Instantiate Cache_Lite
$cache = new Cache_Lite($options);

// Some dummy data to store
$id = 'MyCache';

// Initialize the cache if first time the page is requested
if (!$cache->get($id)) {
  $cache->save('Dummy', $id);
}

// A randomizer...
$random = array(0, 1, 1);
shuffle($random);

// Randomly update the cache
if ($random[0] == 0) {
  $cache->save('Dummy', $id);
}

// Get the time the cache file was last modified
$lastModified = filemtime($cache->_file);

// Issue an HTTP last modified header
header('Last-Modified: ' .
  gmdate('D, d M Y H:i:s', $lastModified) . ' GMT');

// Get client headers - Apache only
$request = getallheaders();

if (isset($request['If-Modified-Since'])) {
  // Split the If-Modified-Since (Netscape < v6 gets this wrong)
  $modifiedSince = explode(';', $request['If-Modified-Since']);

  // Turn the client request If-Modified-Since into a timestamp
  $modifiedSince = strtotime($modifiedSince[0]);
} else {
  // Set modified since to 0
  $modifiedSince = 0;
}

// Compare time the content was last modified with client cache
if ($lastModified <= $modifiedSince) {
  // Save on some bandwidth!
  header('HTTP/1.1 304 Not Modified');
  exit();
}

echo 'The GMT is now ' . gmdate('H:i:s') . '<br />';
echo '<a href="' . $_SERVER['PHP_SELF'] .
  '">View Again</a><br />';
?>

Remember to use the “View Again” link when you run this example (clicking Refresh usually clears your browser’s cache). If you click on the link repeatedly, eventually the cache will be updated; your browser will throw out its cached version and fetch a new page rendered by PHP.

In the above example we used PEAR::Cache_Lite to create a cache file that is updated randomly. We ascertain the file modification time of the cache file with this line:

$lastModified = filemtime($cache->_file);

Technically speaking, this is a hack, as PEAR::Cache_Lite intends its $_file member variable to be private. However, we must use it to get the name of the cache file so that we can fetch its modification time.

// Issue an HTTP last modified header
header('Last-Modified: ' .
  gmdate('D, d M Y H:i:s', $lastModified) . ' GMT');

// Get client headers - Apache only
$request = getallheaders();

if (isset($request['If-Modified-Since'])) {
    // Split the If-Modified-Since (Netscape < v6 gets this wrong)
    $modifiedSince = explode(';', $request['If-Modified-Since']);

    // Turn the client request If-Modified-Since into a timestamp
    $modifiedSince = strtotime($modifiedSince[0]);
} else {
    // Set modified since to 0
    $modifiedSince = 0;
}

Finally, we check to see whether the cache has been modified since the last time the visitor received this page. If it hasn’t, we simply send a Not Modified response header and exit the script, saving bandwidth and processing time by instructing the browser to display its cached copy of the page.

// Compare the time the content was last modified with cache
if ($lastModified <= $modifiedSince) {
  // Save on some bandwidth!
  header('HTTP/1.1 304 Not Modified');
  exit();
}

If you combine the Last-Modified approach with time values that are already available in your application (e.g. the time of the most recent news article, or expiry times from the server side caching system we saw in the last solution), you should be able to take advantage of Web browser caches and save bandwidth, while being able to gather your site’s traffic information and improve its perceived performance.

Be very careful to test any caching performed in this manner, though; if you get it wrong, you may cause your visitors to have permanently out of date copies of your site.