Fixes the canonical redirection URL to have the same hostname as the original requested URL hostname. The best place for using this is when using a reverse proxy (or CDN like Akamai or Fastly) and the WordPress installation is path-based. For example, WP Engine does not support path-based installations where multiple installations have the same …

redirect-canonical.php

<?php
/*
  Plugin Name: WPS Canonical Redirection
  Plugin URI: http://wpsmith.net/
  Description: Fixes the canonical redirection URL to have the same hostname as the original requested URL hostname. The best place for using this is when using a reverse proxy (or CDN like Akamai or Fastly) and the WordPress installation is path-based. For example, WP Engine does not support path-based installations where multiple installations have the same domain (e.g., features.weather.com) but have different paths (e.g., features.weather.com/us-climate-change and features.weather.com/climate25).
  Author: Travis Smith <t@wpsmith.net>
  Version: 0.0.1
 */

add_filter( 'redirect_canonical', 'wps_redirect_canonical', 10 );
/**
 * Filters the canonical redirect URL.
 *
 * Returning false to this filter will cancel the redirect.
 *
 * @param string $redirect_url  The redirect URL.
 * @param string $requested_url The requested URL.
 * @return string $redirect_url Maybe fixed redirect URL.
 */
function wps_redirect_canonical( $redirect_url ) {
	$dest = parse_url( $redirect_url );

	// Since this is the canonical URL, we are assuming the site URL host
	// is the same as the destination URL host.
	$base_url = apply_filters( 'site_or_home_url', site_url() );
	$site = parse_url( $base_url );
	$base_url = preg_replace( '/^https?:\/\//i', '', $base_url );

	// Determine whether we need to change the redirect_url
	// Compare the redirect url host with the site_url
	if ( $dest['host'] !== $site['host'] ) {
		// Replace the host with the host (+path from site_url())
		return str_replace( $dest['host'], $base_url, $redirect_url );
	}

	// Return the same URL
	return $redirect_url;
}