Basic, but very effective, CSS minifier for CodeIgniter - or anything else if you extract the behaviour.

css_minifier.php

<?php
/**
 * @author Dan Bettles <danbettles@yahoo.co.uk>
 * 
 * @license http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ by-nc-sa
 */

if ( ! defined('BASEPATH'))
{
  exit('No direct script access allowed');
}

/**
 * Minifies CSS
 * 
 * @author Dan Bettles <danbettles@yahoo.co.uk>
 */
class Css_minifier {

	public function minify($css)
	{
		$minified_css = $css;

		$minified_css = $this->_normalize_whitespace($minified_css);

		$minified_css = $this->_remove_comments($minified_css);
		$minified_css = $this->_remove_unneeded_whitespace($minified_css);
		$minified_css = $this->_remove_units_from_zeroes($minified_css);
		$minified_css = $this->_collapse_hex_colours($minified_css);

		$pct_reduction = round((strlen($minified_css) / strlen($css)) * 100);
		$message = "/*{$pct_reduction}%*/";

		return $message . PHP_EOL . $minified_css;
	}

	private function _normalize_whitespace($css)
	{
		//Normalize newlines
		$css = preg_replace('/\r\n|\r|\n/', PHP_EOL, $css);

		//Normalize horizontal whitespace
		$css = preg_replace('/[ \t]/', ' ', $css);

		return $css;
	}

	private function _remove_comments($css)
	{
		return preg_replace('{\/\*(?:.*?)\*\/}s', '', $css);
	}

	private function _remove_unneeded_whitespace($css)
	{
		$css = trim($css);

		//Replace multiple, contiguous occurrences of a whitespace character with just the one
		$css = preg_replace('/(\s){2,}/s', '$1', $css);

		//Remove leading and trailing whitespace from lines
		$css = preg_replace('/^[ ]+|[ ]+$/m', '', $css);

		//Remove horizontal whitespace around commas, colons, semi-colons, and braces
		$css = preg_replace('/[ ]*([,:;\{\}])[ ]*/', '$1', $css);

		return $css;
	}

	private function _remove_units_from_zeroes($css)
	{
		return preg_replace('/\b0(?:(?:px|em)\b|%)/i', '0', $css);
	}

	private function _collapse_hex_colours($css)
	{
		$pattern_hex_byte = '[0-9a-fA-F]{2}';

		if (preg_match_all("/(#)({$pattern_hex_byte})({$pattern_hex_byte})({$pattern_hex_byte})/", $css, $matches))
		{
			$replacement_colours = array();

			foreach (array_keys($matches[0]) as $colour_no)
			{
				$original_colour = $matches[0][$colour_no];

				//Proceed to the next colour if we've already replaced this one
				if (array_key_exists($original_colour, $replacement_colours))
				{
					continue;
				}

				$collapsed_colour = $matches[1][$colour_no];  /*(Starts with the prefix)*/

				foreach (array(2, 3, 4) as $byte_no)
				{
					$byte = $matches[$byte_no][$colour_no];

					//Proceed to the next colour if the digits comprising the byte aren't the same as each other
					if ($byte[0] !== $byte[1])
					{
						continue 2;
					}

					$collapsed_colour .= $byte[0];
				}

				//Replace the original colour throughout the CSS
				$css = str_replace($original_colour, $collapsed_colour, $css);

				//Make a note of the colour we just replaced
				$replacement_colours[$original_colour] = $collapsed_colour;
			}
		}

		return $css;
	}
}