failed to work with base_url in Codeigniter 2.2.6

I'm trying to load bootstrap files into Codeigniter using base_url (). My autoload.php file has the following configuration
$autoload['helper'] = array('url');
In my view I put inside the head the following code.

 <link rel="stylesheet" href='<?= base_url("css/bootstrap.css"); ?>'>   

And on my Controller I put the following call:

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