I understand it is not advisable but isn't it possible?
<link rel="stylesheet" type="text/css" href="a_style.css.php">
header("Content-type: text/css; charset=utf-8");
//your php + css code goes here
We have to use PHP to append a query variable to the end of the href
href="a_style.css.php?version=<?php echo $some_number ?>">
otherwise it will use the version in the browsers' cache.
But why bother? That's what classes, ids, etc., are for??