Wordpress Plugin Dynamic Error Page - php

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(); ?>

Answer 2

The solution depends on the content of 404.php file. If this file contains static text, like
_e( 'It seems we can’t find what you’re looking for...', 'twentyeleven' );
you can add your own filter
apply_filters( 'my_404_content', 'Default 404 message' );
and in functions.php (or in plugin)
add_filter( 'my_404_content', 'replace_404_message' );
function replace_404_message($message) {
return 'Error 404 - '.$message;
}
If 404.php uses built-in WP functions to display page content, you should check what filters they are supported.

Answer 3

You might be able to add a the_content filter with a conditional is_404 section:
function content_404($content) {
if (is_404()) {
// do some stuff with $content
}
// no matter what,
return $content;
}
add_filter( 'the_content', 'content_404' );
Note that this does assume that the 404.php page template has a the_content template tag in place.