Place the following code in the style.css.php file, don't forget to include the CSS files you need:

<?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();

Then, in the HTML page, include this PHP file where you include styles:

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