Combine and Compress CSS Files on-the-fly using PHP

style.css.php

<?php
/**
 * Combines multiple CSS files into a single file and compresses it on-the-fly.
 * Usage: 
 * <link rel="stylesheet" type="text/css" href="/css/styles.css.php" />			- 
 *
 */

/* Use https://.. instead of //.. for CDNs or else it will not work */
$css_files = array(
	'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css',
	'styles.css',
	'https://cdnjs.cloudflare.com/ajax/libs/animate.css/3.5.2/animate.min.css',
	'font-awesome.min.css'
);

$buffer = "";
foreach ( $css_files as $file ) {
	$buffer .= file_get_contents( $file );
}
/* 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 month */
header( 'Expires: ' . gmdate('D, d M Y H:i:s', time() + 2592000) . ' GMT' );

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

/* Write everything out */
echo $buffer;