MY_config.php

## config.php
/**
 |--------------------------------------------------------------------------
 | In config.php, you can now use server-based configuration
 |--------------------------------------------------------------------------
 */

/**
 |--------------------------------------------------------------------------
 | Translation table of equivalent server names. For instance, configure
 | yourdomain.com and www2.yourdomain.com the same as www.yourdomain.com
 |--------------------------------------------------------------------------
 */
$config['servers']['equiv'] = array(
        '' => 'www',
        'www2' => 'www',
);

/**
 |--------------------------------------------------------------------------
 | Keys and values specific to each domain
 | You can continue to set other config values normally, and they
 | will apply to all configurations
 |--------------------------------------------------------------------------
 */

$config['servers']['local.dev'] = array(
        'key1' => 'value1',
);

$config['servers']['dev'] = array(
        'key1' => 'value1',
);

$config['servers']['staging'] = array(
        'key1' => 'value1',
);

$config['servers']['www'] = array(
        'key1' => 'value1',
);

## MY_config.php
<?php

class MY_Config extends CI_Config {

    /**
     * Loads in custom server-specific configurations
     *
     * Additional config variables set
     *      server_name - the part of the hostname to the left of '.topleveldomain.com'
     *
     */
    public function load($file = '', $use_sections = FALSE, $fail_gracefully = FALSE) {
        parent::load($file,$use_sections,$fail_gracefully);
        $server_name = $this->server_name();
        $this->config['server_name'] = $server_name;
        if (array_key_exists('servers',$this->config)) {
            $server_configs = $this->config['servers'];
            if (array_key_exists($server_name,$server_configs)) {
                while(list($k,$v) = each($server_configs[$server_name])) {
                    $this->config[$k] = $v;
                }
            }
            unset($this->config['servers']);
        }
    }

    public function dump() {
        print "<pre>";
        print_r($this->config);
        print "</pre>";
    }


    /**
     * Returns the portion of the domain before the [domain_name].[tld]
     * @return string
     */
    public function server_name() {
        $server = $_SERVER['SERVER_NAME'];
        $parts = explode('.',$server);
        array_pop($parts); # .com
        array_pop($parts); # domain
        $server = join('.',$parts);
        if (!array_key_exists('servers',$this->config)) {
            return $server;
        }
        $equiv = $this->config['servers']['equiv'];
        if (array_key_exists($server,$equiv)) {
            $server = $equiv[$server];
        }
        return $server;
    }

    function is_localhost() {
        return ($this->server_name() == 'localhost' || substr($this->server_name(),0,5) == 'local');
    }

    function is_live() {
        return ($this->server_name() == 'www' || $this->server_name() == '' );
    }
}

?>