CodeIgniter with multiple subdomains

I'd suggest executing some code in a important model for the app that gets auto-loaded through the entire app. Maybe call it Site_model.php.

Firstly, lets autoload the model. Open up config/autoload.php and add the model into the list of models to be autoloaded:

$autoload['model'] = array('Site_model');

Now, create the new model calls Site_model. In the construct of the model, add some code along these lines (this is a simply basic example of how to fetch and assign the value of a subdomain. I imagine there are better ways, but for this example, it will do):

class Site_model extends CI_Model {
   var $subDomain = '';
   function __construct() {
      $this->subDomain = array_shift((explode('.', $_SERVER['HTTP_HOST'])));

      // further code which uses the value of subDomain to fetch the correct record from the database.
   }

Now, anytime you want to reference the subdomain in your code, you can simply get it like this:

echo $this->Site_model->subDomain;

This should also work for the admin backend. Oh, and ensure that the model is the first to be auto-loaded, in case any of the other models that are auto-loaded initialise code that is dependent on the site model's values.