Content-type not working in PHP

I have some issues with a PHP file that is not working properly. The Content-type does not get recieved by any browser at all. Firebug interprets the file as text/html instead of css. Here's the file :

<?php
header('Content-Type: text/css; charset=UTF-8');
error_reporting(E_ALL | E_STRICT);
ini_set('display_errors', 'On');
/* CSS goes on from here */

I tested to put a row with echo 'TEST'; before the header line, and was expecting to see the classic "headers already sent" error, but nothing appears!

Normal .css-files are working like a charm however.

What can I do to sort this out?

UPDATE: Did change default_mimetype = "text/html" to default_mimetype = "text/css" in php.ini and all pages got immediately interpreted as css, so there's must be a way to just send css headers for this file :)

The full file from demand of John:

    <?php
    header('Content-Type: text/css; charset=UTF-8');
    echo 'body {background-color: #000000; }';
    ?>

UPDATE #2: Adding ini_set('default_mimetype', 'text/css'); to the PHP file fixes this file, but it doesn't solve the issue that causes this fault...

UPDATE #3: Tested adding AddType text/css .css to both .htaccess and Apache config. Still no luck. Also tested to send headers separated from charset: header('Content-Type: text/css'); - Still no luck...

UPDATE #4: Have reinstalled Apache+PHP at the server to see if the problem goes away, but no. Same old, same old...