Serving Craft 3 from webroot: how to setup your index.php entry file.

index.php

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

// Set path constants
define('CRAFT_BASE_PATH', dirname(__FILE__));
define('CRAFT_VENDOR_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();
}

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

I know this is NOT the recommended setup for Craft. However, some clients just want cheap shared hosting. ¯_(ツ)_/¯

This is how my web root structure looks like for the provided index.php