Change DB config

Hi everyone,

I try to make a login to my DB (directly to the DB, not to a table containing users)

But I don't know how could I change the configuration in the default database

For example this could be the begining : 

PHP Code:

$db['default'] = array(
    'dsn'    => '',
    'hostname' => $tnsname,
    'username' => '',
    'password' => '',
    'database' => '',
    'dbdriver' => 'oci8',
    'dbprefix' => '', 


And then in the controller, change the $username and $passeword with variables so I can login.

Can anyone help me in this case ? any tips ?