FRONTEND_MVC/web/index.php

<?php
const DS = DIRECTORY_SEPARATOR;
// comment out the following two lines when deployed to production
defined('YII_DEBUG') or define('YII_DEBUG', true);
defined('YII_ENV') or define('YII_ENV', 'dev');

require __DIR__ . '/../vendor/autoload.php';
require __DIR__ . '/../vendor/yiisoft/yii2/Yii.php';


$env='prod';
$env_file = __DIR__ . DS . '..' . DS . '.env';

if (file_exists($env_file)) {
    $env = [];
    $lines = explode(PHP_EOL,trim(file_get_contents($env_file)));
    foreach ($lines as $line){
        $pair = explode("=",$line);
        $env[$pair[0]]=$pair[1];
        defined($pair[0]) or define($pair[0], $pair[1]);
    }
    if(\yii\helpers\ArrayHelper::getValue($env,'ENV',false) && $env['ENV']=="dev"){
        $env = "dev";
        defined('YII_DEBUG') or define('YII_DEBUG', true);
        defined('YII_ENV') or define('YII_ENV', 'dev');
        defined('YII_ENV_DEV') or define('YII_ENV_DEV', 'dev');
    }
    if(isset($env['ENV'])){
        $env = $env['ENV'];
    } else{
        $env = "prod";
    }
} else {
    defined('YII_DEBUG') or define('YII_DEBUG', false);
    defined('YII_ENV') or define('YII_ENV', 'prod');
}

if (file_exists(__DIR__ . DS . '..' . DS . 'revision.php')) {
    require_once(__DIR__ . DS . '..' . DS . 'revision.php');
}
if (!defined('REV')) {
    define('REV', 1);
}


$config_path = __DIR__ . DS.'..'.DS.'config'.DS.'env'.DS.$env.'.php';
$config = require($config_path);

$config = require __DIR__ . '/../config/web.php';

(new yii\web\Application($config))->run();