header('HTTP/1.0 404 Not Found'); not doing anything

I have a 404.php file in my site's main directory and I was using header('Location: 404.php'); for a while until someone said that you should use header('HTTP/1.0 404 Not Found'); instead. So I replaced it with that and then added: ErrorDocument 404 /404.php to my apache config file and restarted the server but it doesn't work.

I tried different variations including ErrorDocument 404 404.php and ErrorDocument 404 mywebite/404.php but to no avail.

What I mean by doesn't work is that earlier when using header('Location: 404.php'); it would redirect to the 404.php file but when I replace it with header('HTTP/1.0 404 Not Found'); it seems to just skip over the line and not do anything. It most certainly is not redirecting. The reason I am calling for the redirect is because if a $_GET header value is not recognized the page should 404.

No, it probably is actually working. It's just not readily visible. Instead of just using the header call, try doing that, then including 404.php, and then calling die.

You can test the fact that the HTTP/1.0 404 Not Found works by creating a PHP file named, say, test.php with this content:

<?php

header("HTTP/1.0 404 Not Found");
echo "PHP continues.\n";
die();
echo "Not after a die, however.\n";

Then viewing the result with curl -D /dev/stdout reveals:

HTTP/1.0 404 Not Found
Date: Mon, 04 Apr 2011 03:39:06 GMT
Server: Apache
X-Powered-By: PHP/5.3.2
Content-Length: 14
Connection: close
Content-Type: text/html

PHP continues.

You could try specifying an HTTP response code using an optional parameter:

header('HTTP/1.0 404 Not Found', true, 404);

Since PHP >= 5.4 you can use the following function to set the HTTP status code :

http_response_code(404);

you will still have the control over the content of the response but with the status code being 404 (you can have any status code you want).

http_response_code PHP Documentation

Use these codes for 404 not found.

if(strstr($_SERVER['REQUEST_URI'],'index.php')){
  header('HTTP/1.0 404 Not Found');
  readfile('404missing.html');
  exit();
}

Here 404missing.html is your Not found design page. (it can be .html or .php)

i think this will help you

content of .htaccess

ErrorDocument 404 /error.php
ErrorDocument 400 /error.php
ErrorDocument 401 /error.php
ErrorDocument 403 /error.php
ErrorDocument 405 /error.php
ErrorDocument 406 /error.php
ErrorDocument 409 /error.php
ErrorDocument 413 /error.php
ErrorDocument 414 /error.php
ErrorDocument 500 /error.php
ErrorDocument 501 /error.php

error.php and .htaccess should be put in the same directory [in this case]

Another reason may be if you add any html tag before this redirect. Look carefully, you may left DOCTYPE or any html comment before this line.

After writing

header('HTTP/1.0 404 Not Found');

add one more header for any inexisting page on your site. It may help.

header("Location: http://yoursite/nowhere");
die;