.env is part of the yii-app-yarcode template.
He loads environment variables from .env to getenv(), $_ENV and $_SERVER automagically.
.env file is an easy way to load custom configuration variables that your application needs.
Storing configuration in the environment is one of the tenets of a twelve-factor app.
.env file is added to .gitignore so it is not checked-in the code.
Add variable in .env file
SECRET_KEY="super_seekret_key"
Get variable in php
$key = getenv('SECRET_KEY');