I’m sure there are lots of resources for this, but here is a fairly robust one (WARNING: Don’t do what Mohammed did! I’ll tell you why in a second):

http://sajjadhossain.com/2008/10/27/ssl-https-urls-and-codeigniter/

So, what’s wrong with the approach? He’s editing core files which is a big no no. He should be extending the functionality of the core files. For example, here are the two files I now have in my site:

/application/helpers/MY_url_helper.php
/application/libraries/MY_Config.php

Then, I placed the following code in each of those files, respectively:

MY_url_helper.php

<?php
if( ! function_exists('secure_site_url') )
{
    function secure_site_url($uri = '')
    {
        $CI =& get_instance();
        return $CI->config->secure_site_url($uri);
    }
}

if( ! function_exists('secure_base_url') )
{
    function secure_base_url()
    {
        $CI =& get_instance();
        return $CI->config->slash_item('secure_base_url');
    }
}

if ( ! function_exists('secure_anchor'))
{
    function secure_anchor($uri = '', $title = '', $attributes = '')
    {
        $title = (string) $title;

        if ( ! is_array($uri))
        {
            $secure_site_url = ( ! preg_match('!^\w+://! i', $uri)) ? secure_site_url($uri) : $uri;
        }
        else
        {
            $secure_site_url = secure_site_url($uri);
        }

        if ($title == '')
        {
            $title = $secure_site_url;
        }

        if ($attributes != '')
        {
            $attributes = _parse_attributes($attributes);
        }

        return '<a href="'.$secure_site_url.'">'.$title.'</a>';
    }
}

if ( ! function_exists('secure_redirect'))
{
    function secure_redirect($uri = '', $method = 'location', $http_response_code = 302)
    {
        switch($method)
        {
            case 'refresh'    : header("Refresh:0;url=".secure_site_url($uri));
                break;
            default            : header("Location: ".secure_site_url($uri), TRUE, $http_response_code);
                break;
        }
        exit;
    }
}

if (! function_exists('force_ssl'))
{
    function force_ssl()
    {
        if ($_SERVER["SERVER_PORT"] != 443)
        {
            redirect(str_replace("http://", "https://" , current_url()), "refresh");
        }
    }
}

MY_Config.php

<?php
class MY_Config extends CI_Config {

	function MY_Config()
	{
		parent::CI_Config();
	}

	function secure_site_url($uri = '')
	{
	    if (is_array($uri))
	    {
	        $uri = implode('/', $uri);
	    }

	    if ($uri == '')
	    {
	        return $this->slash_item('secure_base_url').$this->item('index_page');
	    }
	    else
	    {
	        $suffix = ($this->item('url_suffix') == FALSE) ? '' : $this->item('url_suffix');
	       return $this->slash_item('secure_base_url').$this->slash_item('index_page').preg_replace("|^/*(.+?)/*$|", "\\1", $uri).$suffix;
	    }
	}
}

Now, that was easy, wasn’t it? Go forth and do awesome stuff with this.