CodeIgniter - best place to declare global variable

There is a file in /application/config called constants.php

I normally put all mine in there with a comment to easily see where they are:

/**
 * Custom defines
 */
define('blah', 'hello mum!');
$myglobalvar = 'hey there';

After your index.php is loaded, it loads the /core/CodeIgniter.php file, which then in turn loads the common functions file /core/Common.php and then /application/constants.php so in the chain of things it's the forth file to be loaded.

I use a "Globals" class in a helper file with static methods to manage all the global variables for my app. Here is what I have:

globals_helper.php (in the helpers directory)

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

// Application specific global variables
class Globals
{
    private static $authenticatedMemberId = null;
    private static $initialized = false;

    private static function initialize()
    {
        if (self::$initialized)
            return;

        self::$authenticatedMemberId = null;
        self::$initialized = true;
    }

    public static function setAuthenticatedMemeberId($memberId)
    {
        self::initialize();
        self::$authenticatedMemberId = $memberId;
    }


    public static function authenticatedMemeberId()
    {
        self::initialize();
        return self::$authenticatedMemberId;
    }
}

Then autoload this in the autoload.php file

$autoload['helper'] = array('globals');

Lastly, for usage from anywhere within the code you can do this to set the variables:

Globals::setAuthenticatedMemeberId('somememberid');

And this to read it:

Globals::authenticatedMemeberId()

Note: Reason why I left the initialize calls in the Globals class is to have future expandability with initializers for the class if needed. You could also make the properties public if you don't need any kind of control over what gets set and read via the setters/getters.