<?php
  $expire = 365*24*60*60;

  header('Content-Type:text/css');
  header('Expires: ' . date('D, d M Y H:i:s',time()+$expire) . ' GMT');
  header('Cache-Control: max-age=' . $expire);
  header('Pragma:');
	
  ob_start("ob_gzhandler");
	
  if (isset($_GET['stylesheet'])) {
    include $_GET['stylesheet'];
  }
  else {
    include 'reset.css';
    include 'styles.css';
  }	
  ob_flush();
?>

The above will send a content type of text/css indicating to the browser that what it is about to receive is a stylesheet rather than normal plaintext. The following 3 headers specify how the file should be cached so that it does not have to be downloaded everytime. ob_start("ob_gzhandler"); specifies that what follows should be entered into the output buffer as using the GZIP handler so that any content that follows is GZipped. The following if-then-else block is used so that a standard set of stylesheets can be easily included on the same page without having to make multiple HTTP requests. Any stylesheets for the same media type that appear on the same page should really be requested like this instead of seperate link lines in your HTML or instead of @import lines in your CSS file. The final line will then output the GZipped content from the output buffer. If you were able to enable mod_gzip in Apache then you can omit the two lines for starting and flushing the output buffer.

Using the following line in HTML would result in attaching a stylesheet that has the contents of reset.css and styles.css GZipped in a single file:

<link type="text/css" rel="stylesheet" href='./styles/styles.php' media="screen" />

If I did not want those stylesheets to be GZipped and instead wanted to GZip the stylesheet for handheld devices then I would use:

<link type="text/css" rel="stylesheet" href="./styles/styles.php?stylesheet=handheld.css" media="handheld" />

That's all there is to it. It's possible to make a more complex one for doing more such as combining any number of specified stylesheets - but that too should be easy enough to create from the above example. In using this method on this site I have reduced the total filesize of the stylesheets being served from 21.9Kb for normal browsing devices down to 5Kb which now has an average load time of 110ms.