WordPress Plugin: Custom 404 -- Redirect to any page and add a message

custom-404.php

<?php
/*
Plugin Name: Custom 404
Plugin URI: http://orangepineapple.com
Description: Send 404 page to another page or URL.
Version: 2013.08.23
Author: Orangepineapple
Author URI: http://orangepineapple.com
License: GNU GPL
*/

/*  
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

add_action('admin_init', 'op404_register_settings' );
add_action('admin_menu', 'op404_admin_menu');
add_action('template_redirect', 'op404_output_header');
add_action('wp_footer', 'op404_notice');
register_activation_hook(__FILE__, 'op404_activate');
register_deactivation_hook(__FILE__, 'op404_deactivate');
register_uninstall_hook(__FILE__, 'op404_deactivate');

function op404_activate() {
    add_option('op404_type', '301');
    add_option('op404_target', home_url());
    add_option('op404_notice', 'The page you were looking for is no longer available.');
}

function op404_deactivate() {
    delete_option('op404_type');
    delete_option('op404_target');
    delete_option('op404_notice');
}

function op404_register_settings() { 
    register_setting( 'op404_option-group', 'op404_type' );
    register_setting( 'op404_option-group', 'op404_target', 'op404_check_values');
    register_setting( 'op404_option-group', 'op404_notice' );
}

function op404_admin_menu() {
    add_options_page('Custom 404 Settings', "Custom 404", 'manage_options', basename(__FILE__), 'op404_plugin_options');
}

function op404_plugin_options(){

    if (!current_user_can('manage_options'))  {
        wp_die( 'You do not have sufficient permissions to access this page.' );
    }
    
    if (get_option("op404_target") == "") {
        add_option('op404_target', home_url());
    }
    
    ?>
    <div class="wrap">
        <h2>Custom 404 Redirect</h2>
        <form method="post" action="options.php" id="op404_form">
            <?php
            wp_nonce_field('update-options', '_wpnonce');
            settings_fields( 'op404_option-group');
            ?>
            <input type="hidden" name="page_options" value="op404_type, op404_target" />
            <p>
                <select name="op404_type">
                    <option value="301" <?= op404_checked("op404_type", "301") ?>>Enabled</option> 
                    <option value="off" <?= op404_checked("op404_type", "off") ?>>Disabled</option>
                </select>
                <br/><br/>
            </p>
            <p>
                Target Url:<br/>
                <em>Use the entire URL, starting with <strong>http://</strong> .</em>
            </p>
            <p>
                <input type="text" name="op404_target" value="<?= get_option("op404_target") ?>" size="100">
                <br/><br/>
            </p>
            <p>
                Notice to display on redirected page:<br/>
                <em>Leave empty to disable the notice.</em>
            </p>
            <p><textarea name="op404_notice" rows="3" cols="30" style="width: 500px;"><?= get_option("op404_notice") ?></textarea></p>
            <input type="hidden" name="action" value="update" />
            <p class="submit"><input type="submit" name="submit" value="Save Changes" /></p>
        </form>
    </div>
    <?php
}

// Test URL
function op404_check_values($input) {
    if (!op404_is_valid_url($input)) {
        add_settings_error('op404_option-group', 'settings_updated', 'This is not a valid Url', $type = 'error');
    }
    return $input;
}

function op404_checked($checkOption, $checkValue) {
      return get_option($checkOption)==$checkValue ? " selected" : "";
}

// Collect 404 errors and redirect them to URL
function op404_output_header() {

    if ( !is_404() || get_option("op404_type")=="off" ) {
        return;
    }

    if (get_option('op404_target')=="") {
        $target = home_url();
    } else {
        $target = get_option("op404_target");
        $delim = '?';
        if (stristr($target,'?')) {
            $delim = '&';
        }
        $target .= $delim.'404';
    }

    global $wp_query;
    $wp_query->is_404 = true;
    wp_redirect( $target, get_option("op404_type") );
    exit;
}

// Display notice
function op404_notice() {
    if (!isset($_GET['404'])) {
        return;
    }
    
    $notice = get_option("op404_notice");
    if ($notice != '') {
        remove_action( 'init', '_wp_admin_bar_init' );
        global $wp_admin_bar;
        if (isset($wp_admin_bar)) {
            $top = '28';
        } else {
            $top = 0;
        }
        ?>
        <style>
            #op404notice { position: absolute; top: <?= $top ?>px; margin: 0 auto; z-index: 9999; text-align: center; width: 100%; }
            .admin-bar #op404notice { top: 28px; }
            #op404notice span { display: block; width: 600px; margin: 0 auto 15px; padding: 7px; border: 1px solid #d8d8d8; border-top: none; background: #f8f8f8; background: rgba(248,248,248,0.8); border-radius: 0 0 3px 3px; box-shadow: 0 1px 4px -1px rgba(0, 0, 0, .3); }
        </style>
        <div id="op404notice">
            <span><?= $notice; ?></span>
        </div>
        <?php
    }
}

// http://www.roscripts.com/snippets/show/156
function op404_is_valid_url($url) {
    $url = @parse_url($url);

    if ( ! $url) {
        return false;
    }

    $url = array_map('trim', $url);
    $url['port'] = (!isset($url['port'])) ? 80 : (int)$url['port'];
    $path = (isset($url['path'])) ? $url['path'] : '';

    if ($path == '') {
        $path = '/';
    }

    $path .= ( isset ( $url['query'] ) ) ? "?$url[query]" : '';

    if ( isset ( $url['host'] ) AND $url['host'] != gethostbyname ( $url['host'] ) ) {
        if ( PHP_VERSION >= 5 ) {
            $headers = get_headers("$url[scheme]://$url[host]:$url[port]$path");
        } else {
            $fp = fsockopen($url['host'], $url['port'], $errno, $errstr, 30);

            if ( !$fp ) {
                return false;
            }
            fputs($fp, "HEAD $path HTTP/1.1\r\nHost: $url[host]\r\n\r\n");
            $headers = fread ( $fp, 128 );
            fclose ( $fp );
        }
        $headers = ( is_array ( $headers ) ) ? implode ( "\n", $headers ) : $headers;
        return ( bool ) preg_match ( '#^HTTP/.*\s+[(200|301|302)]+\s#i', $headers );
    }
    return false;
}

?>