$file = FCPATH.'applicationconfigassets_fe.php';

            if(is_writable($file)){

                $open = fopen($file, 'W');
                if(!fwrite($open, $frontend)){

                    echo "Could not write file!";

                } else {

                     echo "File written!";
                }

            } else {

                echo "File not writable!";

            }

write_file(FCPATH.'application/config/assets_fe.php', $frontend, 'W');
write_file('application/config/assets_fe.php', $frontend, 'W');
write_file('./application/config/assets_fe.php', $frontend, 'W');

$this->config->load('filename');

$this->config->set_item('item_name', 'item_value');

$this->config->set_item('ftp_frontend', array(...));