base_url() problem

Hello everybody, 

Hi everyone, I have a problem with base_url
I do not understand why when I put <?php echo base_url ()?> asset / css / style.css
I can not reach my destination.
The link is correct in fact if I write the same in the browser I get to the css file.
in practice from what I understand the result I get is: localhost / site / localhost / asset / css / style.css
how can i get:
localhost / assets / css / style.css
using base_url ();