Craft 3 Multisite dynamic mapping based on .env

index.php

<?php
/**
 * Craft web bootstrap file
 */

// Set path constants
define('CRAFT_BASE_PATH', dirname(__DIR__));
define('CRAFT_VENDOR_PATH', CRAFT_BASE_PATH.'/vendor');

// Load Composer's autoloader
require_once CRAFT_VENDOR_PATH.'/autoload.php';

// Load dotenv?
if (file_exists(CRAFT_BASE_PATH.'/.env')) {
    (new Dotenv\Dotenv(CRAFT_BASE_PATH))->load();
}

// Multisite dynamic mapping based on server name defined in `.env`
// Example `.env`: SITES="site1.dev:site1Handle,site2.dev:site2Handle"
if (getenv('SITES')) {
    $sites = explode(',', getenv('SITES'));
    $siteMappings = [];
    foreach($sites as $site) {
        if (strstr($site, ':')) {
            $mapping = explode(':', $site);
            $siteMappings[$mapping[0]] = $mapping[1];
        }
    }
    if (isset($siteMappings[$_SERVER['SERVER_NAME']])) {
        define('CRAFT_SITE', $siteMappings[$_SERVER['SERVER_NAME']]);
    }
}

// Load and run Craft
define('CRAFT_ENVIRONMENT', getenv('ENVIRONMENT') ?: 'production');
$app = require CRAFT_VENDOR_PATH.'/craftcms/cms/bootstrap/web.php';
$app->run();