yii2/docs/guide/structure-entry.md

Entry Scripts

Note: This chapter is under development.

Configuring options in the bootstrap file

For each application in Yii there is at least one bootstrap file: a PHP script through which all requests are handled. For web applications, the bootstrap file is typically index.php; for console applications, the bootstrap file is yii. Both bootstrap files perform nearly the same job:

1. Setting common constants.
2. Including the Yii framework itself.
3. Including Composer autoloader.
4. Reading the configuration file into $config.
5. Creating a new application instance, configured via $config, and running that instance.

Like any resource in your Yii application, the bootstrap file can be edited to fit your needs. A typical change is to the value of YII_DEBUG. This constant should be true during development, but always false on production sites.

The default bootstrap structure sets YII_DEBUG to false if not defined:

defined('YII_DEBUG') or define('YII_DEBUG', false);

During development, you can change this to true:

define('YII_DEBUG', true); // Development only
defined('YII_DEBUG') or define('YII_DEBUG', false);