How to create an error 404 page using PHP?

My file .htaccess handles all requests from /word_here to my internal endpoint /page.php?name=word_here. The PHP script then checks if the requested page is in its array of pages.

If not, how can I simulate an error 404?

I tried this, but it didn't result in my 404 page configured via ErrorDocument in the .htaccess showing up.

header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found");

Am I right in thinking that it's wrong to redirect to my error 404 page?

The up-to-date answer (as of PHP 5.4 or newer) for generating 404 pages is to use http_response_code:

<?php
http_response_code(404);
include('my_404.php'); // provide your own HTML for the error page
die();

die() is not strictly necessary, but it makes sure that you don't continue the normal execution.

What you're doing will work, and the browser will receive a 404 code. What it won't do is display the "not found" page that you might be expecting, e.g.:

Not Found

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

That's because the web server doesn't send that page when PHP returns a 404 code (at least Apache doesn't). PHP is responsible for sending all its own output. So if you want a similar page, you'll have to send the HTML yourself, e.g.:

<?php
header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found", true, 404);
include("notFound.php");
?>

You could configure Apache to use the same page for its own 404 messages, by putting this in httpd.conf:

ErrorDocument 404 /notFound.php

Try this:

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

Create custom error pages through .htaccess file

1. 404 - page not found

 RewriteEngine On
 ErrorDocument 404 /404.html

2. 500 - Internal Server Error

RewriteEngine On
ErrorDocument 500 /500.html

3. 403 - Forbidden

RewriteEngine On
ErrorDocument 403 /403.html

4. 400 - Bad request

RewriteEngine On
ErrorDocument 400 /400.html

5. 401 - Authorization Required

RewriteEngine On
ErrorDocument 401 /401.html

You can also redirect all error to single page. like

RewriteEngine On
ErrorDocument 404 /404.html
ErrorDocument 500 /404.html
ErrorDocument 403 /404.html
ErrorDocument 400 /404.html
ErrorDocument 401 /401.html

Did you remember to die() after sending the header? The 404 header doesn't automatically stop processing, so it may appear not to have done anything if there is further processing happening.

It's not good to REDIRECT to your 404 page, but you can INCLUDE the content from it with no problem. That way, you have a page that properly sends a 404 status from the correct URL, but it also has your "what are you looking for?" page for the human reader.

try putting

ErrorDocument 404 /(root directory)/(error file) 

in .htaccess file.

Do this for any error but substitute 404 for your error.

In the Drupal or Wordpress CMS (and likely others), if you are trying to make some custom php code appear not to exist (unless some condition is met), the following works well by making the CMS's 404 handler take over:

<?php
  if(condition){
    do stuff;
  } else {
    include('index.php');
  }
?>

Immediately after that line try closing the response using exit or die()

header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found");
exit;

or

header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found");
die();

try this once.

$wp_query->set_404();
status_header(404);
get_template_part('404');