自定义404 php页面返回200而不是404

I have set up a custom 404 page and have the following code within the header:

<?php
header("HTTP/1.0 404 Not Found");
?>

Then within my .htaccess file I have the following:

ErrorDocument 404 /404.php

However for some reason when testing the site I am told the page is still returning a 200 instead 404 error. Any suggestions?