<?php

class Container
{
    /**
     * @var \ArrayObject
     */
    protected $permissions;

    public function __construct()
    {
        $this->permissions = $this->permissions ?: new ArrayObject();
    }

    public function addPermission(Permission $permission)
    {
        $this->permissions->append($permission);
    }
}

class Permission
{
    private $id;

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

$container = new Container();
$container->addPermission(new Permission(1));
$container->addPermission(new Permission(2));
$stop = 1;