Классы требуется разъяснение

<?php
class OtherClass extends MyClass {
   public static $my_static = 'статическая переменная';
 
   public static function doubleColon() {
      echo parent::CONST_VALUE . "\n";
      echo self::$my_static . "\n";
   }
}
 
OtherClass::doubleColon();
?>

class SomeClass
{
    protected $db;
 
    public function __construct()
    {
        $this->db = Database::getInstance();
    }
}

class Product extends Entity
{
    public function getAll()
    {
        return $this->db->query('SELECT * FROM `products`');
    }
}

class Entity extends  SomeClass
{
 
}

class Entity
{
    protected $db;
 
    public function __construct()
    {
        $this->db = Database::getInstance();
    }
}
 
 
 
class Product extends Entity
{
    public function getAll()
    {
        return $this->db->query('SELECT * FROM `products`');
    }
}
 
 
class Order extends Entity
{
    public function getAll()
    {
        return $this->db->query('SELECT * FROM `orders`');
    }
}
 
 
 
class User extends Entity
{
    public function getAll()
    {
        return $this->db->query('SELECT * FROM `users`');
    }
}

    public function getAll()
    {
        return $this->db->query('SELECT * FROM `users`');
    }

<?
class Entity
{
    protected $db;
 
    public function __construct()
    {
        $this->db = Database::getInstance();
    }
 
    public function getAll($table)
    {
        return $this->db->query("SELECT * FROM `$table`");
    }
}
 
 
 
class Product extends Entity
{
    public function getByViews()
    {
        return $this->db->query('SELECT * FROM `products` ORDER BY `view_count` DESC LIMIT 20');
    }
}
 
 
class Order extends Entity
{
    public function getByDates($date_start, $date_end)
    {
        return $this->db->query("SELECT * FROM `orders` WHERE `date_create` BETWEEN '$date_start' AND '$date_end'");
    }
}
 
 
 
class User extends Entity
{
    public function checkAuth($user, $password)
    {
        $result = $this->db->query("SELECT * FROM `users` WHERE `login` = '$login' AND `password` = '$password'");
        return (bool) count($result);
    }
}

abstract class Abstr
{}
class Derived extends Abstr
{}
//$a = new Abstr;
$d = new Derived;

class Abstr
{}
class Derived extends Abstr
{}
//$a = new Abstr;
$d = new Derived;