Acquiring data from multiple database source or cluster (simplest arbitrary demonstration)

database.php

<?php

class Database {


    /**
     * The connection resource.
     *
     * @var PDO|null
     */
    protected $connection = null;

    public function __construct(string $dsn, string $username, string $password) {

        $this->connection = new PDO(
            $dsn,
            $username,
            $password,
            $this->dbOptions()
        );

    }

    /** statically create an instance of database
     * class.
     *
     * @param string $dsn
     * @param string $username
     * @param string $password
     * @return Database
     */
    public static function connect(string $dsn, string $username, string $password)
    {
        return new self($dsn, $username, $password);
    }

    protected function action()
    {
        /*
         * [IMPORTANT]
         * Whatever query / action we'd like to perform
         * goes in here.
         * */
        return $this->connection->query('select name from students');
    }

    public function get()
    {
        /*
         * [IMPORTANT]
         * Put whatever logic of getting data here.
         * */
        $result = $this->action()->fetch(PDO::FETCH_ASSOC);
        $this->close();
        return $result;
    }

    /**
     * Closes the database connection
     */
    protected function close()
    {
        $this->connection = null;
    }

    /**
     * Options for PDO connections.
     *
     * @return array
     */
    protected function dbOptions()
    {
      return [
          PDO::ATTR_FECH
      ];
    }
}

index.php

<?php

/*
 * Just a quick hack, it is best to use composer autoloader.
 * */
require_once './db.php';

$data_store = [
    /*
     * since the database name varies by just some unique values at the end
     * we can generate automatically, I will just assume that is true.
     * */
    [
        // 'database' => 'vt1',
        'username' => 'ionware',
        'password' => 'secret'
    ],
    [
        // 'database' => 'vt2',
        'username' => 'ionware',
        'password' => 'secret'
    ],
];

/*
 * Uhm... I might just use the result on the fly, but I can just
 * store it and use it later. So, let's create a result variable (as array)
 * */
$count = 0;

// Would be our database alias.
$db_alias = 'vt';
$dsn = 'mysql:host=localhost;dbname='.$db_alias;

$result = array_reduce($data_store, function($before, $store) use ($count, $dsn) {
    global $count;
    $count++;
    return array_merge($before, [
        'cluster'.$count => Database::connect($dsn.$count, $store['username'], $store['password'])
            ->get()
    ]);
}, []);

var_dump($result);

index2.php

<?php

/*
 * THE LOOP APPROACH
 * Just a quick hack, it is best to use composer autoloader.
 * */
require_once './db.php';

/*
 * Uhm... I might just use the result on the fly, but I can just
 * store it and use it later. So, let's create a result variable (as array)
 * */
$result = [];

// Would be our database alias.
$db_alias = 'vt';
$dsn = 'mysql:host=localhost;dbname='.$db_alias;
$db_name = 'ionware';
$db_pass = 'secret';

/*
 * The Loop approach
 * */
for($i = 1; $i <= 2; /* where to stop*/ $i++) {
    $result = array_merge($result, [
        'db'.$i => Database::connect($dsn.$i, $db_name, $db_pass)
        ->get()
    ]);
}

var_dump($result);

schema.sql

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `vt2`
--

-- --------------------------------------------------------

--
-- Table structure for table `students`
--

CREATE TABLE `students` (
  `id` int(11) NOT NULL,
  `name` varchar(255) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

--
-- Dumping data for table `students`
--

INSERT INTO `students` (`id`, `name`) VALUES
(1, 'John Snow');

--
-- Indexes for dumped tables
--

--
-- Indexes for table `students`
--
ALTER TABLE `students`
  ADD PRIMARY KEY (`id`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `students`
--
ALTER TABLE `students`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=2;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;

Setup the database, update the data_store variable, and run the index.php file.

Let's discuss!