Question

What action or filter can I use in a WordPress plugin to dynamically replace the contents (i.e. not the header or footer) of a 404 error page?

Basically I’m looking for a 404 error page equivalent for the_content filter, which will filter the contents of an existing page.

Thank you for your time.

Note: I know I can manually modify the 404 error page for the current theme, but that is not the effect I am trying to achieve.

Answer 1

From this WordPress Answer: How to control output of custom post type without modifying theme?

Plugin file:

<?php
/*
Plugin Name: Plugin 404 Page
Plugin URI: http://stackoverflow.com/questions/14539884
Description: Use the plugin's template file to render a custom 404.php
Author: brasofilo
Author URI: https://wordpress.stackexchange.com/users/12615/brasofilo
Version: 2013.26.01
License: GPLv2
*/
class Universal_Template
{
    public function __construct()
    {       
        $this->url = plugins_url( '', __FILE__ );   
        $this->path = plugin_dir_path( __FILE__ );
        add_action( 'init', array( $this, 'init' ) );
   }

    public function init() 
    {
        add_filter( 'template_include', array( $this, 'template_404' ) );
    }

    public function template_404( $template ) 
    {
        if ( is_404() )
            $template = $this->path . '/404.php';

        return $template;
    }
}

$so_14539884 = new Universal_Template();

And in the plugin folder, a file named 404.php:

<?php
/**
 * The template for displaying 404 pages (Not Found).
 *
 * @package WordPress
 * @subpackage Twenty_Twelve
 * @since Twenty Twelve 1.0
 */

get_header(); ?>

    <div id="primary" class="site-content">
        MY 404!
    </div><!-- #primary -->

<?php get_footer(); ?>