use of base url and site url in codeigniter code example

Example 1: codeigniter base_url

$this->load->helper('url');

Example 2: dynamic base url codeigniter

defined('BASE_URL') OR define('BASE_URL', (is_https() ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'] . dirname($_SERVER['SCRIPT_NAME']) . '/');
$config['base_url'] = BASE_URL;