Added helpers.md

docs/guide/helpers.md

+Helper Classes
+==============
+
+Yii provides many helper classes to simplify commonly needed coding tasks, such as string/array manipulations,
+HTML code generation, etc. These helper classes are mostly organized under the `yii\helpers` namespace, and
+they 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, $b);
+~~~
+
+Extending Helper Classes
+------------------------
+
+Static classes are typically hard to customize because when you use them, you already hardcode the class names
+in your code, and as a result your customized versions will not get used unless you do some global replacement in your code.
+
+To solve this problem, Yii breaks each helper into two classes: one is base class (e.g. `BaseArrayHelper`)
+and the other the concrete class (e.g. `ArrayHelper`). When you use a helper, you should only use the concrete version.
+
+If you want to customize a helper, e.g., `ArrayHelper`, do the following steps:
+
+1. Name your class the same as the concrete class provided by Yii, including the namespace part, e.g.,
+   `yii\helpers\ArrayHelper`;
+2. Extend your class from the base class, e.g., `class ArrayHelper extends \yii\helpers\BaseArrayHelper`;
+3. In your class, override any method or property as your want, or add new methods or properties;
+4. In your application that you plan to use your own version of the helper class, include the following
+   line of code in the bootstrap script:
+
+```php
+Yii::$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.

docs/guide/index.md

@@ -41,12 +41,13 @@ Database
 
 - [Basics](database-basics.md) - Connecting to a database, basic queries, transactions and schema manipulation
 - [Query Builder](query-builder.md) - Querying the database using a simple abstraction layer
-- [ActiveRecord](active-record.md) - The active record ORM, retrieving and manipulatings records and defining relations
-- [Database Migration](console-migrate.md) - Versioning your database with database migrations
+- [ActiveRecord](active-record.md) - The active record ORM, retrieving and manipulating records and defining relations
+- [Database Migration](console-migrate.md) - Versioning your database with database migration
 
 Developers Toolbox
 ------------------
 
+- [Helper Classes](helpers.md)
 - [Automatic Code Generation](gii.md)
 - [Debug toolbar and debugger](module-debug.md)
 - [Error Handling](error.md)