Allow for nested subdirectories in WordPress Multisite

.htaccess

# BEGIN WordPress Multisite
# Using subfolder network type: https://wordpress.org/documentation/article/htaccess/#multisite

RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
# We are changing this rewrite rule to allow any content to appear before wp-[directories].
# WordPress will take of the rest internally.
# RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^(.+)?/(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

# END WordPress Multisite

sunrise.php

<?php

/** wp-content/sunrise.php **/

if (defined('DOMAIN_CURRENT_SITE') && defined('PATH_CURRENT_SITE')) {
    $current_site = new stdClass();
    $current_site->id = (defined('SITE_ID_CURRENT_SITE') ? constant('SITE_ID_CURRENT_SITE') : 1);
    $current_site->domain = $domain = DOMAIN_CURRENT_SITE;
    $current_site->path = $path = PATH_CURRENT_SITE;

    if (defined('BLOGID_CURRENT_SITE'))
        $current_site->blog_id = constant('BLOGID_CURRENT_SITE');

    $url = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);

    $patharray = (array) explode('/', trim($url, '/'));

    $pathsearch = '';
    $blogsearch = '';

    if (count($patharray)) {
        foreach ($patharray as $pathpart) {
            $pathsearch .= '/' . $pathpart;
            $blogsearch .= $wpdb->prepare(" OR (domain = %s AND path = %s) ", $domain, $pathsearch . '/');
        }
    }

    $current_blog = $wpdb->get_row($wpdb->prepare("SELECT *, LENGTH( path ) as pathlen FROM $wpdb->blogs WHERE domain = %s AND path = '/'", $domain, $path) . $blogsearch . 'ORDER BY pathlen DESC LIMIT 1');

    $blog_id = $current_blog->blog_id;
    $public  = $current_blog->public;
    $site_id = $current_blog->site_id;

    $current_site = corfitz_get_current_site_name($current_site);
}


function corfitz_get_current_site_name($current_site)
{
    global $wpdb;
    $current_site->site_name = wp_cache_get($current_site->id . ':current_site_name', "site-options");
    if (!$current_site->site_name) {
        $current_site->site_name = $wpdb->get_var($wpdb->prepare("SELECT meta_value FROM $wpdb->sitemeta WHERE site_id = %d AND meta_key = 'site_name'", $current_site->id));
        if ($current_site->site_name == null)
            $current_site->site_name = ucfirst($current_site->domain);
        wp_cache_set($current_site->id . ':current_site_name', $current_site->site_name, 'site-options');
    }
    return $current_site;
}

wp-config.php

<?php

// ... rest of config

// Activate sunrise script
define('SUNRISE', TRUE);

/* That's all, stop editing! Happy publishing. */