The majority of components have sensible default settings, so it's unlikely that you'll do a lot of configuration. Still, there are some mandatory configuration settings that you will have to establish, such as the database connection.
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:
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:
```php
defined('YII_DEBUG') or define('YII_DEBUG', false);
```
During development, you can change this to `true`:
The configuration is a large array of key-value pairs. In the above, the array keys are the names of application properties. Depending upon the application type, you can configure the properties of
either [[yii\web\Application]] or [[yii\console\Application]] classes. Both classes extend [[yii\base\Application]].
Note that you can configure not only public class properties, but any property accessible via a setter. For example, to
configure the runtime path, you can use a key named `runtimePath`. There's no such property in the application class, but
since the class has a corresponding setter named `setRuntimePath`, `runtimePath` becomes configurable.
The ability to configure properties via setters is available to any class that extends from [[yii\base\Object]], which is nearly every class in the Yii framework.
The majority of the Yii functionality comes from application components. These components are attached to the application instance via the instance's `components` property:
In the above code, four components are configured: `cache`, `user`, `errorHandler`, `log`. Each entry's key is a component ID. The values are subarrays used to configure that component. The component ID is also used to access the component anywhere within the application, using code like `\Yii::$app->myComponent`.
The configuration array has one special key named `class` that identifies the component's base class. The rest of the keys and values are used
to configure component properties in the same way as top-level keys are used to configure the application's properties.
Each application has a predefined set of components. To configure one of these, the `class` key can be omitted to use the default Yii class for that component. You can check the `registerCoreComponents()` method of the application you are using
Note that Yii is smart enough to only configure the component when it's actually being used: for example, if you configure the `cache` component in your configuration file but never use the `cache` component in your code, no instance of that component will be created and no time is wasted configuring it.