Working with EXTERNAL database with Drupal 7 API - example.

drupal_external_db_connect.php

<?php

//Array with connection credentials. This is the same structured array like in "settings.local.php" file in your Drupal set up.
$db_connection_data = [
    'host' => 'localhost',
    'database' => 'example_db',
    'username' => 'root',
    'password' => '',
    'driver' => 'mysql',
  ];

Database::addConnectionInfo('temporary_db_key', 'default', $db_connection_data);

  //Set active another\external database:
  db_set_active('temporary_db_key');

  /*
    Do anything we want to do with DB via standard Drupal DB API here.
   */

  //It's very important to revert active database to default!
  db_set_active();