// db.php

$host = '127.0.0.1';
$db   = 'test';
$user = 'root';
$pass = '';
$charset = 'utf8';

$dsn = "mysql:host=$host;dbname=$db;charset=$charset";
$opt = [
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES   => false,
];
$pdo = new PDO($dsn, $user, $pass, $opt);


// controller.php

require_once 'db.php'

$userModel = new UserModel($db);
$user = $userModel->findOne($_GET['id']);


// UserModel.php

class UserModel
{

    private $db;

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

    public function findOne($id)
    {
        $stmt = $this->db->prepare("SELECT FROM user WHERE id = ?");
        $stmt->execute([$id]);
        return $stmt->fetch();
    }
}