setup.php

<?php
/**
 * Multisite setup for sub-directories or path based
 * URLs for subsites.
 *
 * DO NOT EDIT UNLESS YOU KNOW WHAT YOU ARE DOING!
 */

use Grav\Common\Filesystem\Folder;

if (!isset($_SERVER['REQUEST_URI'])) {
    return [];
}

// Get relative path from Grav root.
$path = isset($_SERVER['PATH_INFO'])
   ? $_SERVER['PATH_INFO']
   : Folder::getRelativePath($_SERVER['REQUEST_URI'], ROOT_DIR);

// Extract name of subsite from path
$name = Folder::shift($path);
$folder = "sites/{$name}";
$prefix = "/{$name}";

if (!$name || !is_dir(ROOT_DIR . "user/{$folder}")) {
    return [];
}

// Prefix all pages with the name of the subsite
$container['pages']->base($prefix);

return [
    'environment' => $name,
    'streams' => [
        'schemes' => [
            'user' => [
               'type' => 'ReadOnlyStream',
               'prefixes' => [
                   '' => ["user/{$folder}"],
                   // 'theme' => ["user/themes"]
               ]
            ],
            'plugins' => [
                'type' => 'ReadOnlyStream',
                'paths' => [
                    "user/plugins"
                ]
            ],
            'themes' => [
                'type' => 'ReadOnlyStream',
                'paths' => [
                    "user/themes"
                ]
            ],
        ]
    ]
];