composer.json

{
    "require": {
        "vlucas/phpdotenv": "^2.5@dev",
        "jeremykendall/php-domain-parser": "~2.0"
    }
}

index.php

<?php

require_once dirname(dirname(__FILE__)) . '/init.php';

// Base configuration
define('YII_DEBUG', env('YII_APP_DEBUG', true));
define('YII_ENV', env('YII_APP_ENV', 'production'));

$domain = new Pdp\Parser( (new Pdp\PublicSuffixListManager)->getList() );
$subdomain = $domain->parseUrl($_SERVER['HTTP_HOST'])->toArray()['subdomain'];

switch($subdomain)
{

    default:
    case null:

        $config = getConfig('index');

    break;

    case env('YII_APP_SUBDOMAIN', 'app'):

        $config = getConfig('app');

    break;

    case env('YII_ADMIN_SUBDOMAIN', 'admin'):

        $config = getConfig('admin');

    break;
}

$application = new yii\web\Application($config);

$application->run();

init.php

<?php

require_once "vendor/autoload.php";

// Load environment variable
$env = new Dotenv\Dotenv(dirname(__FILE__));
$env->load();

/**
 * Returns the base path
 *
 * @param string $filename
 * @return string
 */
function basePath($filename = null)
{
	$path = dirname(__FILE__);

	return is_null($filename) ? $path : $path . '/' . $filename;
}

/**
 * Gets the value from the environment file
 *
 * @param string $key 
 * @param mixed $val Default value
 */
function env(string $key, $val = null)
{
	return ! is_null($return = getenv($key)) ? $return : $val;
}

/**
 * Gets the correct config for the environment
 *
 * @param string $envrionment
 * @return array
 */
function getConfig(string $environment) : array
{
	$config = [];
	$merge_config = [];

	$config = require_once(basePath('config/yii2/' . $environment . '/main.php'));

	foreach(glob(basePath('config/yii2/*.php')) as $file) {
		$merge_config[basename($file, ".php")] = require_once $file;
	}

	return array_merge_recursive($config, $merge_config);
}

$aliases = [
	// aliases here
];


array_walk_recursive($aliases, function($val, $key) {
	Yii::setAlias($key, $val);
});

yii

#!/usr/bin/env php
<?php

require_once 'init.php';

@define('YII_DEBUG', env('YII_APP_DEBUG', true));
@define('YII_ENV', env('YII_APP_ENV', 'production'));

$config = getConfig('console');

/**
 * Configurations which are not needed
 *
 */
unset($config['components']['request']);
unset($config['components']['errorHandler']);

$application = new yii\console\Application($config);

$exitCode = $application->run();
exit($exitCode);