Appending numbers to url do not break the link

That’s an old problem, and I have never looked deep enough explain it. But I have a plugin for that, written a long time ago. It still works, but keep the age of the code in mind when reading it. 🙂 I didn’t change much for this repost.

<?php
/*
Plugin Name: T5 Canonical Permalink
Plugin URI: http://toscho.de/2010/wordpress-plugin-canonical-permalink/
Description: Removes illegal numeric suffixes from the request URI.
Version: 0.3
Author: Thomas Scholz
Author URI: http://toscho.de
Created: 04.04.2010
*/

add_action('wp', 't5_canonical_request');
/**
 * WordPress allows URIs with any numeric suffix, e.g.:
 * /canonical-page-or-postname/12345/
 * This functions performs a simple check and redirects
 * to the canonical URI if neccessary.
 *
 * @return void
 */
function t5_canonical_request()
{
    global $page, $post;

    // post, page, attachment, preview
    if ( ! is_singular() or is_preview() )
    {
        return;
    }

    $permalink = get_permalink();

    // We don't have access to the number of sub pages here.
    // So we have to hack.
    $max_pages = substr_count(
        $post->post_content, '<!--nextpage-->') + 1;

    if ( 1 < $page and $page <= $max_pages )
    {
        /*
         * Handle different permalink settings, eg:
         * /%year%/%postname%.html or
         * /%year%/%postname%/
         */
        $rev_perma_struct = strrev(get_option('permalink_structure'));

        if ( "https://wordpress.stackexchange.com/" != $rev_perma_struct[0] )
        {
            $permalink .= "/$page";
        }
        else
        {
            $permalink .= "$page/";
        }
    }

    $host_uri       = 'http'
                    . ( empty ( $_SERVER['HTTPS'] ) ? '' : 's' )
                    . '://' . $_SERVER['HTTP_HOST'];
    $canonical_path = str_replace($host_uri, '', $permalink);

    if ( ! empty ( $_GET ) )
    {
        global $wp;
        // Array
        $allowed = $wp->public_query_vars;

        $out_arr = array();

        foreach ( $_GET as $k => $v )
        {
            if ( in_array($k, $allowed ) )
            {
                $out_arr[] = $k . ( empty ( $v ) ? '' : "=$v" );
            }
        }

        if ( ! empty ( $out_arr ) )
        {
            $canonical_path .= '?' . implode('&', $out_arr);
        }
    }

    if ( $canonical_path == $_SERVER['REQUEST_URI'] )
    {
        return;
    }
    // Debug current result:
    #print '<pre>' . var_export($canonical_path, TRUE) . '</pre>';

    // Change it or return 'false' to stop the redirect.
    $canonical_path = apply_filters(
        't5_canonical_path',
        $canonical_path
    );

    if ( FALSE != $canonical_path )
    {
        header('Location: ' . $permalink, true, 301);
        die("<a href="https://wordpress.stackexchange.com/questions/70992/$permalink">$permalink</a>");
    }

    return;
}