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+############################
+Creating Core System Classes
+############################
+
+Every time CodeIgniter runs there are several base classes that are
+initialized automatically as part of the core framework. It is possible,
+however, to swap any of the core system classes with your own versions
+or even extend the core versions.
+
+**Most users will never have any need to do this, but the option to
+replace or extend them does exist for those who would like to
+significantly alter the CodeIgniter core.**
+
+.. note:: Messing with a core system class has a lot of implications, so
+	make sure you know what you are doing before attempting it.
+
+System Class List
+=================
+
+The following is a list of the core system files that are invoked every
+time CodeIgniter runs:
+
+-  Benchmark
+-  Config
+-  Controller
+-  Exceptions
+-  Hooks
+-  Input
+-  Language
+-  Loader
+-  Log
+-  Output
+-  Router
+-  URI
+-  Utf8
+
+Replacing Core Classes
+======================
+
+To use one of your own system classes instead of a default one simply
+place your version inside your local application/core directory::
+
+	application/core/some-class.php
+
+If this directory does not exist you can create it.
+
+Any file named identically to one from the list above will be used
+instead of the one normally used.
+
+Please note that your class must use CI as a prefix. For example, if
+your file is named Input.php the class will be named::
+
+	 class CI_Input {  }
+
+Extending Core Class
+====================
+
+If all you need to do is add some functionality to an existing library -
+perhaps add a function or two - then it's overkill to replace the entire
+library with your version. In this case it's better to simply extend the
+class. Extending a class is nearly identical to replacing a class with a
+couple exceptions:
+
+-  The class declaration must extend the parent class.
+-  Your new class name and filename must be prefixed with MY\_ (this
+   item is configurable. See below.).
+
+For example, to extend the native Input class you'll create a file named
+application/core/MY_Input.php, and declare your class with::
+
+	 class MY_Input extends CI_Input {  }
+
+Note: If you need to use a constructor in your class make sure you
+extend the parent constructor::
+
+	 class MY_Input extends CI_Input {      function __construct()     {         parent::__construct();     } }
+
+**Tip:** Any functions in your class that are named identically to the
+functions in the parent class will be used instead of the native ones
+(this is known as "method overriding"). This allows you to substantially
+alter the CodeIgniter core.
+
+If you are extending the Controller core class, then be sure to extend
+your new class in your application controller's constructors.
+
+::
+
+	class Welcome extends MY_Controller {      function __construct()     {         parent::__construct();     }      function index()     {         $this->load->view('welcome_message');     } }
+
+Setting Your Own Prefix
+-----------------------
+
+To set your own sub-class prefix, open your
+application/config/config.php file and look for this item::
+
+	$config['subclass_prefix'] = 'MY_';
+
+Please note that all native CodeIgniter libraries are prefixed with CI\_
+so DO NOT use that as your prefix.