An environment file is a configuration file that contains environment-specific settings for a CakePHP application. The environment file is typically used to store sensitive information, such as database credentials and API keys, that should not be stored in version control.

To use an environment file in a CakePHP application, you will need to do the following:

1. Create the environment file: Create a file named .env in the root directory of your CakePHP application. You can use the following format to specify key-value pairs in the file:

KEY=VALUE

For example, you can specify the database credentials like this:

DB_HOST=localhost
DB_USERNAME=root
DB_PASSWORD=password

2. Load the environment file: Add the following code to your application’s config/bootstrap.php file to load the environment file:

$dotenv = new \Cake\Core\Dotenv(ROOT);
$dotenv->load();

This will load the environment file and make the key-value pairs available as environment variables.

3. Use the environment variables: You can access the environment variables in your application by using the env() function. For example, you can use the following code to access the database credentials from the environment file:

$host = env('DB_HOST');
$username = env('DB_USERNAME');
$password = env('DB_PASSWORD');

By using an environment file and the env() function, you can store sensitive information in a secure location and access it in your CakePHP application.