Connecting to Multiple Databases Within Drupal

In the settings.php file, $databases is an array composed of multiple database connection strings.
Here’s the default syntax, specifying a single connection:

array(
  'driver' => 'mysql',
  'database' => 'databasename',
  'username' => 'username',
  'password' => 'password',
  'host' => 'localhost',
  'port' => 3306,
  'prefix' => 'myprefix_',
);

As an example, you might have two databases, the default database (in this case named D7) and a
legacy database as defined here.

$databases = array (
  'default' =>
  array (
    'default' =>
    array (
      'driver' => 'mysql',
      'database' => 'd7',
      'username' => 'username',
      'password' => 'userpassword',
      'host' => 'localhost',
      'port' => '',
      'prefix' => '',
    ),
  ),
  'legacy' =>
  array (
    'default' =>
    array (
      'driver' => 'mysql',
      'database' => 'legacydatabase',
      'username' => 'legacyusername',
      'password' => 'legacyuserpassword',
      'host' => '122.185.22.1',
      'port' => '6060',
    ),
  ),
);

When you need to connect to one of the other databases in Drupal, you activate it by its key name
and switch back to the default connection when finished:

// Get some information from a non-Drupal database.
db_set_active('legacy');
$result = db_query("SELECT * FROM ldap_user WHERE uid = :uid", array(':uid' => $user->uid));

// Switch back to the default connection when finished.
db_set_active('default');