Critical css for PHP sites using grunt-penthouse (could use Penthouse's node module to exact same effect)

gruntfile.js

// gruntfile.js
-----------------------
// make sure you're actually running a php server locally first!
// Output a critical css file in same folder with same name as php file, just add '.css' extensions
// This makes it super easy to find from php.
penthouse: {
	work : {
		css : 'pathToMyFullCss.css',
		url : 'localhost:8000/work/index.php',
		outfile : 'work/index.php.css',
		width : 1300,
		height : 900
	},
	... other pages
}

header.php

$pagefile = $_SERVER['DOCUMENT_ROOT'] . htmlspecialchars($_SERVER['PHP_SELF']);
$critcssfile = $pagefile . ".css";
if (file_exists($critcssfile)) { ?>
	<!-- inline critical css -->
  <style><?php include($critcssfile);?></style>
  <!-- use loadCSS to load full CSS async, from: https://github.com/filamentgroup/loadCSS -->
  <script>
		function loadCSS(n,e,o,d){"use strict";var t=window.document.createElement("link"),i=e||window.document.getElementsByTagName("script")[0],l=window.document.styleSheets;return t.rel="stylesheet",t.href=n,t.media="only x",d&&(t.onload=d),i.parentNode.insertBefore(t,i),t.onloadcssdefined=function(e){for(var o,d=0;d<l.length;d++)l[d].href&&l[d].href.indexOf(n)>-1&&(o=!0);o?e():setTimeout(function(){t.onloadcssdefined(e)})},t.onloadcssdefined(function(){t.media=o||"all"}),t}
		loadCss('pathToMyFullCss.css');
	</script>
	<!-- provide a no JS fallback to get the full css -->
	<noscript><link rel='stylesheet' href='pathToMyFulLCss.css' /></noscript>
} else { ?>
	<!-- no critical css for page - just load full css in render blocking way.. -->
	<link rel='stylesheet' href='pathToMyFullCss.css' />
<?php } ?>

I figured out that the term 'work' could be anything. So, you need to keep it unique for all pages and can do multiple pages at once.