Взаимодействие классов

<?php
session_start();
 
require_once('app/route.php');
 
$route = new Route;
$route->run();
 
require_once('app/db_connect.php');
 
$db = new DB;
$db->connect();
?>

<?php
class DB {
 
    protected $connection;
 
    public function сonnect() {
        $this->connection = new mysqli("localhost", "root", "", "mos_db");
 
        if(!$this->connection) {
            throw new Exception("Connect error");
        }
 
        return $this->connection;
    }
 
}
?>

<?php
class User {
 
    public function getUsers() {
        $result = array();
        
        $res = $this->connection->query("SELECT * FROM `users`");
        while($row = $res->fetch_assoc()) {
            $result[] = $row;
        }
 
        return $result;
    }
 
}
?>

class User
{
   public function __construct(ConnectionInterface $db)
   {
 
   }
}

interface ConnectionInterface
{
    public function getConnection();
}

<?php
class DB implements ConnectionInterface 
{
 
    protected $connection;
 
    public function __construct()
    {
        $this->connection = new mysqli("localhost", "root", "", "mos_db");
 
        if(!$this->connection) {
            throw new Exception("Connect error");
        }
 
        return $this->connection;
    }
   
    public function getConnection()
    {
        return $this->connection;
    }
}
?>

class DB
{
    private static $instance = null;
 
    public static function connection()
    {
        if (null === self::$instance)
        {
            self::$instance = new mysqli("localhost", "root", "", "mos_db");;
        }
        
        return self::$instance;
    }
    
    private function __clone() {}
    private function __construct() {}
}
 
class Entity {
    private $connection;
    
    public function __construct() {
        $this->connection = DB::connection();
    }
}
 
class User extends Entity {
 
    public function getUsers() {
        $result = array();
        
        $res = $this->connection->query("SELECT * FROM `users`");
        while($row = $res->fetch_assoc()) {
            $result[] = $row;
        }
 
        return $result;
    }
 
}