yii2-app-basic/app/config/db.php

<?php

return [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=' . getenv('DB_HOST') . ';dbname=' . getenv('DB_NAME'),
    'username' => getenv('DB_USER'),
    'password' => getenv('DB_PASS'),
    'charset' => 'utf8'
];