MY_Controller and autoload

[eluser]JonoB[/eluser]
This one has me completely baffled. I set up a test CI project on my localhost. As part of this, I created a file in application/core/my_controller.php as follows:

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

Code:

class MY_Controller extends CI_Controller
{
    function __construct()
    {
        parent::__construct();

        $this->output->enable_profiler(TRUE);
        
        $this->session->set_userdata(array('company_id' => 1));
        
        if ( ! $this->tank_auth->is_logged_in())
        {
            redirect('/auth');
        }
    }
}

?>

And my controllers then extend this as follows:

Code:

class Products extends MY_Controller
{
    function __construct()
    {
        parent::__construct();
    }
  
    function index()
    {      
        //do something here      
    }
}

And this works absolutely 100% fine.

I then moved my files onto my live webserver, and kept getting a WSOD when trying to load any controller/function. No error message, no 404, nothing. I eventually tracked this down to My_Controller class - it was not autoloading on my live server. So, to double check I put the following line right at the top of each of my controllers:

Code:

require_once '/home/something/public_html/application/core/my_controller.php';

And, voila, all controllers now work fine.

So, why the hell does this work fine in my localhost (xampp running PHP 5.3.1) and not on my live server (PHP Version 5.3.3)? Am I missing something really obvious here?