Dao.php

<?php
abstract class Dao {
   protected $db;

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

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

   public function getDb() {
      return $this->db;
   }
}

Entity.php

<?php
abstract class Entity {
   public $id;
}

Usage.php

<?php

$db = new PDO(...);
$userDao = new UserDao($db);
$user = $userDao->getById($id);

User.php

<?php
class User extends Entity {
   public $name;
   public $password;
}

UserDao.php

<?php
class UserDao extends Dao {
   public function getById($id) {
        $stmt = $this->db->query("SELECT id,name,password FROM users WHERE id=:id");
        $stmt->bindValue(':id', $id, PDO::PARAM_INT);
        $stmt->execute();
        $user = $stmt->fetchObject('User');
        $stmt->closeCursor();
        return $user;
   }
}