In the PHP world, this usually mean a .env file in the root of your project. Most modern frameworks, such as Laravel and Symfony, come with built in support for a .env file.
In some cases, you might simply prefer to use a more standard environment file over the way in which configuration is currently provided by your project.
If not, make sure you include vendor/autoload.php in any file you wish to access your .env file variables from. You can now create a .env file in the root of your project, alongside your composer.json file and populate it as follows.