PHP Code igniter comes with URL Helper to assist you in working with application URLs. This helper contains bunch of methods to access URLs and in order to use them, you must first load the URL helper either inside controller or auto load it using autoload.php file.

You will need to set your base URL in application/config/config.php

To load URL Helper use this statement,

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

Or add this below line to 'config/autoload.php'

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

Open config.php and find below code

$config['base_url'] = '';

Now Replace With 

$root = (isset($_SERVER['HTTPS']) ? "https://" : "http://") . $_SERVER['HTTP_HOST'];
$root .= str_replace(basename($_SERVER['SCRIPT_NAME']), '', $_SERVER['SCRIPT_NAME']);
$config['base_url'] = $root;

OR

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