<?php
  header('Content-type: text/css');
  ob_start("compress");
  function compress($buffer) {
    /* remove comments */
    $buffer = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $buffer);
    /* remove tabs, spaces, newlines, etc. */
    $buffer = str_replace(array("
", "\r", "\n", "\t", '  ', '    ', '    '), '', $buffer);
    return $buffer;
  }
	
  /* your css files */
  include('master.css');
  include('typography.css');
  include('grid.css');
  include('print.css');
  include('handheld.css');

  ob_end_flush();
?>

http://www.catswhocode.com/blog/3-ways-to-compress-css-files-using-php

Combine multiple css files into 1 file that has been stripped of whitespace and comments.

CSS Combine and Compress

PHP