gae-app.php and app.yaml setup for WordPress Multisite in subfolders on Google App Engine Standard

app.yaml

# App Engine runtime configuration
runtime: php74
instance_class: F4
  
# Defaults to "serve index.php" and "serve public/index.php". Can be used to
# serve a custom PHP front controller (e.g. "serve backend/index.php") or to
# run a long-running PHP script as a worker process (e.g. "php worker.php").
entrypoint: serve gae-app.php

# Defines static handlers to serve WordPress assets
handlers:

# serves static files from the subfolders. i.e. /fr/
- url: /([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*\.(htm.*$|html.*$|css.*$|js.*$|ico.*$|jpg.*$|png.*$|gif.*$)) 
  static_files: \2 
  upload: /([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*\.(htm.*$|html.*$|css.*$|js.*$|ico.*$|jpg.*$|png.*$|gif.*$)) 
  secure: always
  
- url: /(.*\.(htm|html|css|js))
  static_files: \1
  upload: .*\.(htm|html|css|js)$
  secure: always

- url: /wp-content/(.*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg))
  static_files: wp-content/\1
  upload: wp-content/.*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg)$
  secure: always

- url: /(.*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg))
  static_files: \1
  upload: .*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg)$
  secure: always

- url: /wp-includes/images/media/(.*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg))
  static_files: wp-includes/images/media/\1
  upload: wp-includes/images/media/.*\.(ico|jpg|jpeg|png|gif|woff|ttf|otf|eot|svg)$
  secure: always

- url: /.*
  secure: always
  script: auto

gae-app.php

<?php

/**
 * This file handles all routing for a WordPress project running on App Engine.
 * It serves up the appropriate PHP file depending on the request URI.
 *
 * @see https://cloud.google.com/appengine/docs/standard/php7/runtime#application_startup
 */

/**
 * Function to return a PHP file to load based on the request URI.
 *
 * @param string $full_request_uri The request URI derivded from $_SERVER['REQUEST_URI'].
 */
function get_real_file_to_load($full_request_uri)
{
    $request_uri = @parse_url($full_request_uri)['path'];

    // Redirect /wp-admin to /wp-admin/ (adds a trailing slash)
    if ($request_uri === '/wp-admin') {
        header('Location: /wp-admin/');
        exit;
    }

    // Redirect /subfolder/wp-admin to /subfolder/wp-admin/ (adds a trailing slash)
    if ($request_uri === '/fr/wp-admin') {
        header('Location: /fr/wp-admin/');
        exit;
    }

    // Serve up "index.php" when /wp-admin/ is requested
    if ($request_uri === '/wp-admin/') {
        return '/wp-admin/index.php';
    }

    // Fix for the network page redirecting to main page
    if ($request_uri === '/wp-admin/network/') {
        return '/wp-admin/network.php';
    }

    // Serve up "index.php" when /subfolder/wp-admin/ is requested
    if ($request_uri === '/fr/wp-admin/') {
        return '/wp-admin/index.php';
    }

    // Fix for login redirect heading back to the subfolder page 
    if ($request_uri === '/fr/wp-login') {
        return '/wp-login.php';
    }

    // Fix for subfolder content paths
    if (strpos($request_uri, '/fr/wp-') !== false) {
        return str_replace("/fr/wp-", "/wp-", $request_uri);
    }

    // Load the file requested if it exists
    if (is_file(__DIR__ . $request_uri)) {
        return $request_uri;
    }

    // Send everything else through index.php
    return '/index.php';
}

// fixes b/111391534
$_SERVER['HTTPS'] = $_SERVER['HTTP_X_APPENGINE_HTTPS'];

// Loads the expected WordPress framework file
// (e.g index.php, wp-admin/* or wp-login.php)
$file = get_real_file_to_load($_SERVER['REQUEST_URI']);

// Set the environment variables to reflect the script we're loading
// (in order to trick WordPress)
$_SERVER['DOCUMENT_URI']    = $_ENV['DOCUMENT_URI']    = $file;
$_SERVER['PHP_SELF']        = $_ENV['PHP_SELF']        = $file;
$_SERVER['SCRIPT_NAME']     = $_ENV['SCRIPT_NAME']     = $file;
$_SERVER['SCRIPT_FILENAME'] = $_ENV['SCRIPT_FILENAME'] = __DIR__ . $file;

require __DIR__ . $file;