URL Helper not found in CSS

Morning & Afternoon all,

I've got a style sheet called custom.css.php that contains this:

<?php header("Content-type: text/css; charset: UTF-8"); ?>

body,
html {
   width: 100%;
   height: 100%;
}

... more styling stuff ...

.archive-banner {
   background: url(<?= base_url("assets/media/images/hero-banner.jpg"); ?>) no-repeat center center;
   background-size: cover;
}



When I go to any page however, the formatting is all a bit odd. If I view the source of the style sheet then I can see this error:

.archive-banner {
   background: url(<br />
<b>Fatal error</b>:  Call to undefined function base_url() in <b>/var/www/public/projects/ida-ci/assets/styles/custom.css.php</b> on line <b>47</b><br />

I'm assuming that this error means that the URL helper isn't getting pulled into the custom.css.php file. But my autoload.php contains this: 

$autoload['helper'] = array('url');

Do I need to explicitly specify that the style sheet needs to include this helper too? Any ideas would be greatly appreciated. Cheers!