$config['base_url'] = 'http://www.mydomain.com';

Do this below:

$config['base_url'] = (isset($_SERVER['HTTPS']) ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'];

Also if you want that even if users visit your site using HTTP protocol like http://www.mydomain.com,
it should redirect to HTTPS (https://www.mydomain.com), add the code below to your .htaccess file.

RewriteCond %{SERVER_PORT} ^80$
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]

.htaccess example:

RewriteEngine On
RewriteBase /

RewriteCond %{SERVER_PORT} ^80$
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [L]

<?php
function root_url(){

	$domain = $_SERVER['HTTP_HOST'];
	$protocol_array = explode('/',$_SERVER['SERVER_PROTOCOL']);
	$protocol = strtolower($protocol_array[0]);

	$path = str_replace( basename($_SERVER['SCRIPT_FILENAME']), '', $_SERVER['PHP_SELF'] );

	$url = $protocol.'://'.$domain.$path;

	return $url;
}

function root_dir(){

	$dir = dirname($_SERVER['SCRIPT_FILENAME']).'/';

	return $dir;
}
?>

It is secure from the following:
- SQL injection.
- Multiple random login attempt
- Cross Site Scripting

This approach are using the following OOP features:
- Class
- Abstract Class
- Inheritance (Extending a class)
- Visibility (public, private, protected)
- Interfaces

MVC features:
- Model model.php
- View
— view/login.php
— view/defaults.php
- Controller controller.php

MVC flowchart hierarchy:

/* config.php */
abstract class Config {
}
/* database.php parent config.php */
interface intDB {
}
abstract class DB extends Config implements intDB {
}
/* methods.php parent database.php */
interface intMethods {
}
class Methods extends DB implements intMethods {
}

/* model.php parent methods.php */
class modelBlog extends Methods {
}
/* controller.php parent model.php */
class controllerBlog extends modelBlog {
}
/* view/login.php */
/* view/defaults.php */

/*
execute index.php
new controllerBlog();
*/

Installation:
- Unzip blog_20110213.zip to your htdocs
- Import database blog.sql using database name blog
- Run it with this url http://localhost/blog/index.php
- Login default are username: user and password: user