class DB
{
    public $db;

    public function __construct()
    {
        $this->db = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
    }
}

class UserDB extends DB
{
    public function getById($id)
    {
        $stmt = $this->db->prepare("SELECT * FROM user WHERE id = :id");
        $stmt->execute(['id' => $id]);
        return $stmt->fetch();
    }
}

$userDB = new UserDB();
$user1 = $userDB->getById(101);