On-the-fly CSS Compression

css-compress.php

<?php
/**
 * On-the-fly CSS Compression
 * Copyright (c) 2009 and onwards, Manas Tungare.
 * Creative Commons Attribution, Share-Alike.
 *
 * In order to minimize the number and size of HTTP requests for CSS content,
 * this script combines multiple CSS files into a single file and compresses
 * it on-the-fly.
 *
 * To use this in your HTML, link to it in the usual way:
 * <link rel="stylesheet" type="text/css" media="screen, print, projection" href="/css/compressed.css.php" />
 */

/* Add your CSS files to this array (THESE ARE ONLY EXAMPLES) */
$cssFiles = array(
  "ads.css",
  "formatting.css",
  "pagesections.css",
  "print.css",
  "screen.css",
  "sidebar.css"
);

/**
 * Ideally, you wouldn't need to change any code beyond this point.
 */
$buffer = "";
foreach ($cssFiles as $cssFile) {
  $buffer .= file_get_contents($cssFile);
}

// Remove comments
$buffer = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $buffer);

// Remove space after colons
$buffer = str_replace(': ', ':', $buffer);

// Remove whitespace
$buffer = str_replace(array("\r\n", "\r", "\n", "\t", '  ', '    ', '    '), '', $buffer);

// Enable GZip encoding.
ob_start("ob_gzhandler");

// Enable caching
header('Cache-Control: public');

// Expire in one day
header('Expires: ' . gmdate('D, d M Y H:i:s', time() + 86400) . ' GMT');

// Set the correct MIME type, because Apache won't set it for us
header("Content-type: text/css");

// Write everything out
echo($buffer);
?>

Why you don't use trim(); ?

$buffer = preg_replace('/[\t\r\n]+/', '', $buffer);
$buffer = preg_replace('/[\s]{2,}/', ' ', $buffer);
$buffer = str_replace(': ', ':', $buffer);

I forked this to fix an issue it had. It would break space-separated values when the developer has used more than one space to separate the values. Pertinent code is here:

// Remove whitespace
$buffer = str_replace(array("\r\n", "\r", "\n", "\t"), '', $buffer);

// Collapse adjacent spaces into a single space
$buffer = ereg_replace(" {2,}", ' ',$buffer);

// Remove spaces that might still be left where we know they aren't needed
$buffer = str_replace(array('} '), '}', $buffer);
$buffer = str_replace(array('{ '), '{', $buffer);
$buffer = str_replace(array('; '), ';', $buffer);
$buffer = str_replace(array(', '), ',', $buffer);
$buffer = str_replace(array(' }'), '}', $buffer);
$buffer = str_replace(array(' {'), '{', $buffer);
$buffer = str_replace(array(' ;'), ';', $buffer);
$buffer = str_replace(array(' ,'), ',', $buffer);

@manas, if compressing multiple stylesheets from different locations. And, some stylesheets contains links to fonts and images. How to handle this?

Adding to this, you can shorten hex colors like #aabbcc to #abc with

$buffer = preg_replace("/#([0-9a-fA-F])\\1([0-9a-fA-F])\\2([0-9a-fA-F])\\3/", "#$1$2$3", $buffer);

I've combined all the space removal str_replace by @brentonstrine and added a bit more to include >, ~ and : to the detected characters

// Remove spaces that might still be left where we know they aren't needed
$buffer = preg_replace( "/\s*([\{\}>~:;,])\s*/", "$1", $buffer );
// Remove last semi-colon in a block
$buffer = preg_replace( "/;\}/", "}", $buffer );

Might be in the extremes:

// Convert CSS content icons to glyphs
function css_content_hex_to_glyph( $matches ) {
    return html_entity_decode( '&#x' . trim( $matches[1], '\\' ) . ';', 0, 'UTF-8' );
}
$buffer = preg_replace_callback( "/(?<=content:[\"'])(\\\[0-9a-fA-F]+)/", 'css_content_hex_to_glyph', $buffer );