yii2/docs/guide/helpers.md

Helper Classes

Yii provides many helper classes to help simplify some common coding tasks, such as string/array manipulations, HTML code generation. These helper classes are organized under the yii\helpers namespace and are all static classes (meaning they contain only static properties and methods and should not be instantiated). You use a helper class by directly calling its static method, like the following,

use yii\helpers\ArrayHelper;

$c = ArrayHelper::merge($a, $b);

Extending Helper Classes

Static classes are typically hard to customize because when you use them, you already hardcode the class names in your code, and as a result your customized versions will not get used unless you do some global replacement in your code.

To solve this problem, Yii breaks each helper into two classes: one is base class (e.g. BaseArrayHelper) and the other the concrete class (e.g. ArrayHelper). When you use a helper, you should only use the concrete version.

If you want to customize a helper, e.g., ArrayHelper, do the following steps:

1. Name your class the same as the concrete class provided by Yii, including the namespace part, e.g., yii\helpers\ArrayHelper;
2. Extend your class from the base class, e.g., class ArrayHelper extends \yii\helpers\BaseArrayHelper;
3. In your class, override any method or property as your want, or add new methods or properties;
4. In your application that you plan to use your own version of the helper class, include the following line of code in the bootstrap script:

Yii::$classMap['yii\helpers\ArrayHelper'] = 'path/to/ArrayHelper.php';

The Step 4 above will instruct Yii class autoloader to load your version of the helper instead of the one included in the Yii distribution.

Tip: You can also use Yii::$classMap to replace ANY core Yii class, not necessarily helper classes, with your own customized version.