Differences in session variable declaration

Hi, I have the following hook

PHP Code:

class LanguageLoader
{
  function initialize() {
    $ci =& get_instance();
    $ci->load->helper('language');

    $site_lang = $_SESSION['site_lang']; 
    //$site_lang = $ci->session->site_lang;
    if ($site_lang) {
        $ci->lang->load('general', $site_lang);
    } else {
        $ci->lang->load('general', 'english');
    }
  }

} 


With the line

PHP Code:

$site_lang = $_SESSION['site_lang']; 


I get the following error

PHP Code:

A PHP Error was encountered
Severity: Notice

Message: Undefined index: site_lang

Filename: hooks/LanguageLoader.php

Line Number: 12

Backtrace:

File: C:\xampp\htdocs\availability-calendar\application\hooks\LanguageLoader.php
Line: 12
Function: _error_handler

File: C:\xampp\htdocs\availability-calendar\index.php
Line: 315
Function: require_once 


but replacing it with

PHP Code:

$site_lang = $ci->session->site_lang; 


resolves the problem. I'm curious to know what the issue was though. Thanks.