Problem with apache mod_rewrite and 404 pages

I have a custom 404 error page, and a mod_rewrite rule. If I access a page that does not exist, I get my 404 error page. My problem is that if I issue a 404 header from my php page, it does not open my 404 page, instead, I get this:

Not Found

The requested URL /index.php was not found on this server.

Additionally, a 404 Not Found error was encountered while trying
to use an ErrorDocument to handle the request.

This is my .htaccess:

RewriteEngine on
ErrorDocument 404 /errors/404.php
RewriteRule ^[A-Z开发者_运维知识库a-z0-9]{8}/$ /index.php

This is my redirect from /index.php that will 404 only if the key does not exist. The $key is obtained from parsing the URL (e.g. http://localhost/aKeYCoDE/):

<?php
if (!key_exists($key)){
    header('HTTP/1.0 404 Not Found');
    exit;
}
?>

I am expecting it to redirect to my 404 page.

UPDATE:

It is definitely something about the fact that I am calling 404 from a page that was rewritten (/index.php). If I create a dummy page: /redirect.php, and then do nothing but issue the 404 from there, I get my custom 404 page. But, if I write a mod_rewrite rule for it, and try to access it that way, I get the default 404 error page.

I found the answer - the problem was my assumption that a header 404 from php would redirect to a 404 page. That was wrong. The Apache server can issue 404 for pages that do not exist, but for pages that do exist, i.e., are being served by the php page, the 404 response goes to the browser.

This thread was of a similar issue: http://www.webmasterworld.com/forum88/10955.htm

To make php do what I want it to do (issue the 404 when key does not exist), I need to include the 404 page from php:

<?php
if (!key_exists($key)){
    include($_SERVER["DOCUMENT_ROOT"]."/errors/404.php");
    exit;
}
?>