Using PHP to serve dynamic CSS

This page in

<link> element:

<link rel="stylesheet" type="text/css" href="css/dynamic_css.php?color=<?php echo preg_replace('/[^a-z]/i', '', $_GET['color']); ?>">

"CSS" (served by css/dynamic_css.php):

<?php header('Content-Type: text/css'); ?>
body { color: <?php echo preg_replace('/[^a-z]/i', '', $_GET['color']); ?>; }

CSS (as seen by the browser):

body { color: currentColor; }