apache conf errordocument不适用于php中声明的404标头

I am issuing a 404 in my coupon.php page:

<?php
$id=$_GET['cid'];
$rs=mysqli_fetch_assoc(mysqli_query($scx_dbh,"select * from locations where locid=$id"));
if($rs==NULL){
  header("HTTP/1.0 404 Not Found");
}

Inside my apache config httpd.conf I have the following declared:

ErrorDocument 404 /404.html

However when i go to the page that issues the 404 it does not load the error document but just shows a browser error stating a 404. It only seems to load when a page does not exist, not when i issue a 404 in php.

Any help would be appreciated