You can’t make both PHP and client-side assets use the same base URL, unless you use PHP to echo a base URL variable or constant to the page.
The usual approach is to have a bootstrap file that you include on every page, and define your base URL and other site-wide variables in there.
bootstrap.php:
<?php
define('BASE_URL', 'http://example.com');
index.php:
<?php
include('bootstrap.php');
?>
<!DOCTYPE html>
<html>
<head>
<!-- // -->
<link rel="stylesheet" href="<?php echo BASE_URL; ?>/css/styles.css" />
</head>
<body>
<!-- // -->
</body>
</html>
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