How do environment variables and .env files help manage different configuration settings across development, staging, and production environments?

Environment variables let you store configuration values, such as database credentials or API keys, outside of your actual application code, and a .env file provides a convenient way to define these variables locally during development, with the important practice of never committing this file to version control since it often contains sensitive credentials, while production environments typically set these same environment variables directly through the hosting platform's own configuration mechanism instead of relying on an actual .env file.

// .env file (never committed to version control)
DB_HOST=localhost
DB_PASSWORD=secret123

// Accessing in PHP
$dbHost = $_ENV['DB_HOST'] ?? getenv('DB_HOST');