How to change the ENVIRONMENT type

If I can change it whether

PHP Code:

define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'testing');  // testing, production, development 


Or

PHP Code:

define('ENVIRONMENT', 'development');  // testing, production, development 


What is the benefit of the following condition

PHP Code:

isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV']