class Db {

    protected $link;
    private $host;
    private $user;
    private $pass;
    private $dbName;

    public function __construct($host, $user, $pass, $db) {
        $this->host = $host;
        $this->user = $user;
        $this->pass = $pass;
        $this->dbName = $db;
        $this->connect();
    }

public function connect() {

        try
        {

            $this->link = new PDO("mysql:host=" . $this->host .
                    ";dbname=" .  $this->dbName . ";charset=utf8mb4",  $this->user,  $this->pass );
            $this->link->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->link->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

        } catch (PDOException $exc)
        {
            $this->setMessage($exc->getMessage());
        }
    }
}
$db = new Db(HOST, USER,PASS,DB)

class Main extends DB {

    public function __construct() {

    }
    /** App methods bellow*/
    }

$db2 = new Db(HOST2,USER2,PASS2,DB2);
class User extends DB {

    public function __construct() {

    }
    /** App methods bellow*/
    }

class Main extends DB
{

    public function __construct()
    {
        parent::__construct();
    }
    // ...
}

class Main extends DB
{

    public function __construct($host, $user, $pass, $db)
    {
        parent::__construct($host, $user, $pass, $db);
    }
    // ...
}

class Main
{
    protected $db;

    public function __construct($db)
    {
        $this->db = $db;
    }
    // ...
}

$mainDb = new Db(HOST, USER,PASS,DB)
$main = new Main($mainDb);