In Laravel, you can define environment variables in the .env file located in the root directory of your Laravel project. The .env file is used to store configuration settings, including sensitive information like database credentials, API keys, and other environment-specific configurations.

To define an environment variable in Laravel:

1. Open the .env file in the root of your Laravel project.

2. Add a new line to the file in the format VARIABLE_NAME=value. For example:

    DB_HOST=localhost
    DB_PORT=3306
    DB_DATABASE=my_database
    DB_USERNAME=my_username
    DB_PASSWORD=my_password
   

   Here, DB_HOST, DB_PORT, DB_DATABASE, DB_USERNAME, and DB_PASSWORD are environment variables related to the database configuration.

3. Save the .env file.

4. To use these environment variables in your Laravel application, you can access them using the env() function provided by Laravel. For example, in your config/database.php file, you can access the database configuration like this:

    'mysql' => [
        'driver' => 'mysql',
        'host' => env('DB_HOST', 'localhost'),
        'port' => env('DB_PORT', '3306'),
        'database' => env('DB_DATABASE', 'my_database'),
        'username' => env('DB_USERNAME', 'my_username'),
        'password' => env('DB_PASSWORD', 'my_password'),
        // other database configuration...
    ],
   

   The second parameter in the env() function is the default value that will be used if the environment variable is not set.

   Remember to never commit your .env file to version control, as it may contain sensitive information. Instead, you should commit a .env.example file with placeholder values and provide instructions on how to set up the actual .env file. Developers can copy the .env.example file and configure it according to their environment.