Multi Site domain for CI 3

index.php

 // index.php
$domain = $_SERVER['SERVER_NAME'];
$site_id = 0; // default
switch ($domain)
{
    case 'www.domain1.com':
        $site_id = 1;
        break;
    case 'www.domain2.com':
        $site_id = 2;
        break;
    default:
        show_error('No Site Found');
        break;
}
define('SITE_ID', $site_id);