reverse-proxy.php

<?php
/**
 * Plugin Name: Reverse Proxy for WordPress Multisite
 * Plugin URI: https://limecuda.com/
 * Description: Reverse proxy setup for a WordPress multisite in subdirectory mode
 * Version: 1.0.0
 * Author: Josh Mallard
 * Author URI: https://limecuda.com
 *
 * @package reverse-proxy-wp-multisite
 *
 * Props to James Paden of https://instrumentalapp.com for the original code we based this off of.
 * @link     https://pressable.com/blog/2015/10/15/reverse-proxy-plugin-for-using-a-hosted-wordpress-site-in-a-subdirectory/
 */

// Change to match the desired subfolder, no leading or trailing slash.
if ( ! defined( 'RP_SUBFOLDER' ) ) {
	define( 'RP_SUBFOLDER', 'blog' );
}

if( ! defined( 'RP_FINAL_URL' ) ) {
	define( 'RP_FINAL_URL', 'https://www.finalsite.com/' );
}

if( ! defined( 'RP_ADMIN_URL' ) ) {
	define( 'RP_ADMIN_URL', 'http://adminurl.com' );
}

/**
 * Conditional to check if we're on the login page for the originating site
 *
 * @return      boolean
 * @since       1.0.0
 */
function lc_is_login_page() {
	return in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ), true );
}

/**
 * If the request is coming from the Reverse Proxy, replace all URLs being output by WordPress
 * with the final site URL
 *
 * @link     http://stackoverflow.com/questions/772510/wordpress-filter-to-modify-final-html-output
 */
if ( ! lc_is_login_page() ) {
	if ( defined( 'WP_SITEURL' ) &&  RP_ADMIN_URL === WP_SITEURL ) {

		ob_start();
		add_action('shutdown', function() {
			$final = '';
			$levels = count( ob_get_level() );
			for ( $i = 0; $i < $levels; $i++ ) {
				$final .= ob_get_clean();
			}
			$final = str_replace( 'http://' . $_SERVER['HTTP_HOST'], RP_FINAL_URL . RP_SUBFOLDER, $final );
			echo $final;
		}, 0 );

	}
}

add_filter( 'wpseo_canonical', 'lc_persistent_canonical' );
/**
 * Make this a universal change since we want this applied everywhere
 * even if we aren't viewing view Reverse Proxy.
 *
 * Replace the "final_url" with the correct Final URL for the site
 *
 * @param      $canonical string
 */
function lc_persistent_canonical( $canonical ) {
	$canonical = str_replace( network_site_url(), RP_FINAL_URL . RP_SUBFOLDER . '/', $canonical );
	return $canonical;
}