Change database file by means of a controller

This is database.php in the config

 $tnsname = '(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST =        
 192.168.0.59)(PORT = 3021))
 (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = xe)))';

$db['default'] = array(
     'dsn'  => '',
     'hostname' => $tnsname,
     'username' => 'user',
     'password' => '******',
     'database' => '',
     'dbdriver' => 'oci8',
     'dbprefix' => '',
     'pconnect' => FALSE,
     'db_debug' => (ENVIRONMENT !== 'production'),
     'cache_on' => FALSE,
     'cachedir' => '',
     'char_set' => 'utf8',
     'dbcollat' => 'utf8_general_ci',
     'swap_pre' => '',
     'encrypt' => FALSE,
     'compress' => FALSE,
     'stricton' => FALSE,
     'failover' => array(),
     'save_queries' => TRUE
 );

How do I change the value of the variable $ tnsname through a controller ?

When the user authenticates in the system and chooses a code in the login, this code will indicate which value goes in that variable ( $ tnsname )