How to trigger 404 for custom query var?

Question

How can you trigger a 404 when using custom query vars? I've got re-write rules written for a custom query var, but if you were to request a URL for the query var that should technically be a 404 it returns a normal WP page, but no content, because nothing technically exists for the URL.

Answer 1

There is a method specifically for this:

function my_parse_query( $wp_query ) {
    if ( $wp_query->get( 'my_custom_var' ) > 42 )
        $wp_query->set_404();
        status_header( 404 );
}
add_action( 'parse_query', 'my_parse_query' );

That should load the 404.php template in your theme, if you have it. Otherwise, it will fall back to index.php.

This will also trigger a HTTP 404 status code.

Answer 2

Add a 404.php to your theme and look if it is called.

Sample 404.php

<!doctype>
<title>404</title>
<p>404 – Mwah!</p>

What is more important: the real HTTP headers, here the status code. You can see it with your browsers developer tools under network. Here is a screen shot for http://wordpress.stackexchange.com/404 in Opera Dragonfly:

See full size image

If you still get a status code 200, look into the global variables $wp and $wp_query. They will tell you more details about the fetched resource.