PHP: CSS Asset Pipeline

css.php

<?php

$files = [
    // Insert Files Here
];

$str = '';

function combine_file_contents($filename, $str)
{
    if (file_exists($filename)) {
        $str .= file_get_contents($filename) . "\r\n";
        return $str;
    } else {
        return $str;
    }
}

foreach($files as $style) {
    $str = combine_file_contents($style, $str);
}

$ETag = sha1($str);

/* Set Headers */
header('Content-Type: text/css');
header('ETag: ' . $ETag);

/* Output CSS */
echo $str;

Simply add css files you wish to source into the $files array, then you can <link rel="stylesheet" src="css.php" type="text/css" />