Access design to MySql database [closed]

I've been investigating good implementations that allow easy adaptability to my db connection. I would like to receive feed backs and suggestions about improvements to this design. For example: how would you implement more than one database in this model, would it be better to use an abstract class for the database, if so, how would you do it? I am in a project that has several phases, in the future there may be a need to change / add DB's and tables and I do not want to have to redo the structure. File Contents:

config.php:

$db1Config = array(
   "dbName" => "master_db",
   "dbUser" => "master_user",
   "dbPassword" => "master_pass",
   "dbHost" => "master_host"
);

Class DB, DB.php:

class DB {

   private static $_instance = Null;
   private $_db = Null;

   public function __construct($dbConfig) {

      try {
         $this->_db = new PDO('mysql:host=' .$dbConfig["dbHost"]. ';dbname=' .$dbConfig["dbName"], $dbConfig["dbUser"], $dbConfig["dbPassword"]);
        $this->_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

      } catch (Exception $e) {
        $error = $e->getMessage();
        //echo $error;
        echo 'Updating database. Try again in a few moments. Sorry';
        die();
      }
   }

   public static function getInstance($dbConfig) {
      if(!isset(self::$_instance)) {
         self::$_instance = new DB($dbConfig);
      }
      return self::$_instance;
   }

   public function select_data($table, $where, $column, $upOrDown)
      ...
   }

   ...
}

Class Service, Service.php

class Service {

   private $_db = Null;

   public function __construct($dbConfig) {
      $this->_db = DB::getInstance($dbConfig);
   }

   public function service_exists($servId) {
      $servExists = $this->_db->select_data('services', array('id', '=', $servId), Null, Null);

      if (count($servExists) > 0) {
         return True;
      }

      return False;
   ...
}

index.php

$service = new Service($db1Config);
if($service->service_exists($servId) {
   ...
}
...