css.php joins and minifies css files from html template into cached.css and controls Cache-Control headers based on mtime of all files engaged

css.php

<?php

header('Content-Type: text/css');

$cacheFile = __DIR__.'/css/cached.css';
$listFile = __DIR__.'/css/cached-list.txt';
$htmlFile = __DIR__.'/tmpl/web/html5.twig';

$htmlTime = filemtime($htmlFile);
$cacheTime = file_exists($cacheFile)? filemtime($cacheFile) : 0;


# Update list of cached css files if it does not exist yet or if html file changed

if (!file_exists($listFile) || filemtime($listFile) < $htmlTime)
{
	$html = file_get_contents($htmlFile);
	preg_match('|<head.+</head\s*?>|is', $html, $m);
	$body = $m[0];

	# http://regexr.com?32712
	$regex = <<<REGEX
/<link[^>]+href\s*=\s*('|")(?P<link1>[^'"]+?\.css)('|")|{{\s*css\s*\(\s*('|")(?P<link2>[^)'"]+)('|")\s*\)\s*}}/is
REGEX;
	preg_match_all($regex, $body, $matches, PREG_SET_ORDER);
	$files = [];
	foreach($matches as $match)
	{
		if (isset($match['link1']) && !empty($match['link1']))
			$link = $match['link1'];
		elseif (isset($match['link2']) && !empty($match['link2']))
			$link = $match['link2'];
		else
			continue;
		if (strtolower(substr($link, -4)) == '.css' && file_exists($filePath = __DIR__.$link))
			$files[] = $filePath;
	}
	if (file_exists($listFile))
		unlink($listFile);
	file_put_contents($listFile, join("\n", $files));
	$forceUpdate = true;
}

if (!file_exists($cacheFile))
	$forceUpdate = true;

# ?update=1 in DEBUG mode forces update, even if cached.css already exists

if (!isset($forceUpdate) && file_exists($cacheFile) && array_key_exists('update', $_GET))
{
	$configFile = __DIR__.'/proj/config.php';
	if (file_exists($configFile) && is_readable($configFile))
	{
		require $configFile;
		if (defined('DEBUG') && DEBUG)
			$forceUpdate = true;
	}
}

# Update if filemtime() for any of files from the list is changed
if (!isset($forceUpdate))
{
	if (!isset($files))
		$files = file($listFile, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
	$maxTime = 0;
	foreach($files as $f)
		$maxTime = max($maxTime, filemtime($f));
	if ($maxTime > $cacheTime)
		$forceUpdate = true;
}

# Generate cached.css

if (isset($forceUpdate))
{
	require(__DIR__.'/proj/utils/CSSmin.php');
	$filters = array (
		"ImportImports"                 => false,
		"RemoveComments"                => true,
		"RemoveEmptyRulesets"           => true,
		"RemoveEmptyAtBlocks"           => true,
		"ConvertLevel3AtKeyframes"      => array("RemoveSource" => false),
		"ConvertLevel3Properties"       => true,
		"Variables"                     => true,
		"RemoveLastDelarationSemiColon" => true
	);
	$plugins = array (
		"Variables"                     => true,
		"ConvertFontWeight"             => true,
		"ConvertHslColors"              => true,
		"ConvertRgbColors"              => true,
		"ConvertNamedColors"            => true,
		"CompressColorValues"           => true,
		"CompressUnitValues"            => true,
		"CompressExpressionValues"      => true
	);

	if (!isset($files))
		$files = file($listFile, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);

	$css = '';
	foreach($files as $file)
	{
		# Try to find .min file - use already minified files if they are found; for example, it is good for files of frameworks
		if (preg_match('|\.min\.css$|is', $file))
			$minified = file_get_contents($file);
		else
		{
			$minFile = preg_replace('|\.css$|is', '.min.css', $file, 1);
			if (file_exists($minFile))
				$minified = file_get_contents($minFile);
			else
				$minified = CssMin::minify(file_get_contents($file), $filters, $plugins);
		}
		$css .= $minified."\n";
	}

	if (file_exists($cacheFile))
		unlink($cacheFile); // unlink clears the cache automatically
	file_put_contents($cacheFile, $css);
	$cacheTime = filemtime($cacheFile);
}

# Output

if (array_key_exists('HTTP_IF_MODIFIED_SINCE', $_SERVER) && strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $cacheTime)
{
	header('HTTP/1.1 304 Not Modified');
	die();
}
header('Last-Modified: '.gmdate('D, d M Y H:i:s', $cacheTime).' GMT');
$offset = 3600 * 24 * 30;
header("Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . ' GMT');
header("Cache-Control: public, max-age=3600, must-revalidate");
die(file_get_contents($cacheFile));