Reorganized guide.

docs/guide/extend-customizing-core.md

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+Helper Classes
+==============
+
+Yii provides many classes that help simplify  common coding tasks, such as string or array manipulations,
+HTML code generation, and so forth. 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 one of its static methods:
+
+
+```
+use yii\helpers\ArrayHelper;
+
+$c = ArrayHelper::merge($a, $b);
+```
+
+Extending Helper Classes
+------------------------
+
+To make helper classes easier to extend, Yii breaks each helper class into two classes: a base class (e.g. `BaseArrayHelper`)
+and a concrete class (e.g. `ArrayHelper`). When you use a helper, you should only use the concrete version, never use the base class.
+
+If you want to customize a helper, perform the following steps (using `ArrayHelper` as an example):
+
+1. Name your class the same as the concrete class provided by Yii, including the namespace: `yii\helpers\ArrayHelper`
+2. Extend your class from the base class: `class ArrayHelper extends \yii\helpers\BaseArrayHelper`.
+3. In your class, override any method or property as needed, or add new methods or properties.
+4. Tell your application to use your version of the helper class by including the following line of code in the bootstrap script:
+
+```php
+Yii::$classMap['yii\helpers\ArrayHelper'] = 'path/to/ArrayHelper.php';
+```
+
+Step 4 above will instruct the Yii class autoloader to load your version of the helper class instead of the one included in the Yii distribution.
+
+> Tip: You can use `Yii::$classMap` to replace ANY core Yii class with your own customized version, not just helper classes.