The $ this-config-set_item does not change the value of the variable in the config.php - CODEIGNITER

The $this->config->set_item does not change the value of the variable in config.php

Code in the controller:

$premisa1 = $this->input->post('numero_premisas');
$this->config->set_item('premisass',$premisa1);

Code in config.php :

$config['premisass'] = '1';

In the config.php start the variable in 1 but with the instruction of the controller that value should be updated and it does not