Added support for $_SERVER['CI_ENV'] in index.php

Remember this is entirely optional, nothing will change if you do not which to use Multiple Environments just like right now. If you DO set CI_ENV you can manipulate the switch that controls error reporting, etc, so set it to "production" on your live site to hide errors from users. If you don't require this logic you can remove it, or change it entirely to check HTTP_HOST for environment subdomains, or check IP address, etc.

index.php

@@ -42,7 +42,7 @@
  *
  * NOTE: If you change these, also change the error_reporting() code below
  */
-	define('ENVIRONMENT', 'development');
+	define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'development');
 /*
  *---------------------------------------------------------------
  * ERROR REPORTING

user_guide_src/source/changelog.rst

@@ -19,6 +19,7 @@ Release Date: Not Released
 -  General Changes
 
    -  PHP 5.1.6 is no longer supported. CodeIgniter now requires PHP 5.2.4.
+   -  ``$_SERVER['CI_ENV']`` can now be set to control the ``ENVIRONMENT`` constant.
    -  Added an optional backtrace to php-error template.
    -  Added Android to the list of user agents.
    -  Added Windows 7, Android, Blackberry and iOS to the list of user platforms.

user_guide_src/source/general/environments.rst

@@ -11,10 +11,16 @@ when "live".
 The ENVIRONMENT Constant
 ========================
 
-By default, CodeIgniter comes with the environment constant set to
+By default, CodeIgniter comes with the environment constant set to use 
+the value provided in ``$_SERVER['CI_ENV']``, otherwise defaults to 
 'development'. At the top of index.php, you will see::
 
-	 define('ENVIRONMENT', 'development');
+	define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'development');
+
+This server variable can be set in your .htaccess file, or Apache 
+config using `SetEnv <https://httpd.apache.org/docs/2.2/mod/mod_env.html#setenv>`_. 
+Alternative methods are available for nginx and other servers, or you can 
+remove this logic entirely and set the constant based on the HTTP_HOST or IP.
 
 In addition to affecting some basic framework behavior (see the next
 section), you may use this constant in your own development to