PHP wrapper for servers where mod_expires and/or mod_deflate are not available. Files routed through this wrapper will be served gzip-compressed, with an ETag and with a "far future expires" header.

web-page-envelope.php

<?php

/**
 * Page delivery wrapper for servers where mod_expires and/or mod_deflate are
 * not available.
 *
 * Route page requests through this wrapper (via mod_rewrite for example) and
 * they will be served with far future expires, etags and will be gzip
 * compressed.
 *
 * Works with both programmatically generated and static files.
 *
 * @param URI $_REQUEST['URI'] URI/URL of the text file to process
 *
 * @copyright Copyleft: http://www.gnu.org/copyleft/copyleft.html
 * @author Jonathan M. Hollin <darkblue@sdf.lonestar.org>
 */


// CONFIGURE
// ---------

// URI to custom 404 page. Edit as necessary (static file, NOT a script).
define('CUSTOM_404', __DIR__ . '/errors/404.html');
// Expiry time of file (from now)
define('EXPIRY_TIME', (365.25 * 24 * 60 * 60)); // seconds
// END CONFIGURATION


// Sanity Check
if ($_REQUEST['uri']) $URI = filter_var($_REQUEST['uri'], FILTER_SANITIZE_URL);
// Generate a 404 error if the URI is bad
if (!$URI) notFound();

// Target file ($target) - absolute path
$target = __DIR__ . '/' . $URI;
// Can the URI be resolved?
if ( file_exists($target) ) {
	// Trying read as a file
	$buffer = file_get_contents($target);
} else {
	// Trying read as URL
	$buffer = file_get_contents($URI);
}
// Get the MIME type of the resource
$MIMEType = ($buffer) ? getMIMEType($buffer) : false;

// The Process
if ($buffer === false) {
	// The $buffer has no content. 404 Error!
	notFound();
} else {
	// The $buffer has content.
	// Turn on the gzip handler, do so gracefully
	if ( ini_get('zlib.output_compression') ) {
		// Don't do anything if compression is already enabled.
		ob_start();
	} else {
		if (!ob_start('ob_gzhandler')) ob_start();
	}
	// Header pre-flight
	$cacheTime = filemtime($target);
	$gmt_mtime = gmdate('D, d M Y H:i:s', $cacheTime ) . ' GMT';
	// Send the headers
	if ($MIMEType) header('Content-type: ' . $MIMEType);
	header('Last-Modified: ' . $gmt_mtime, true);
	header('Expires: ' . gmdate("D, d M Y H:i:s", $cacheTime + EXPIRY_TIME) . ' GMT', true);
	header('Cache-Control: no-store, no-cache, must-revalidate, proxy-revalidate', true);
	header('Cache-Control: post-check=0, pre-check=0', FALSE);
	header('Etag: ' . md5($target));
	// Send the page
	echo $buffer;
	// Send the contents of the output buffer and turn buffering off
	// (we're done!)
	ob_end_flush();
}

function notFound() {
	// Send 404 headers
	header($_SERVER['SERVER_PROTOCOL'] . ' 404 Not Found');
	header('Status: 404 Not Found');
	if ( file_exists(CUSTOM_404) ) {
		// Load and display the custom 404 page
		@readfile(CUSTOM_404);
	} else {
		// Display basic 404 error message
		echo '<h1>404 Not Found</h1>';
		echo 'The page that you have requested could not be found.';
	}
	die();
}

function getMIMEType($resource) {
	$finfo = new finfo(FILEINFO_MIME);
	if ($resource) return $finfo->buffer($resource);
}

Background: http://darkblue.sdf.org/weblog/post/web-page-envelope