<?php

class Organisation {

	private $name;
	private $departments = [];

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

	public function addDepartment(Department $dep) {
		$this->departments[] = $dep;
	}


}

class Department {
	public function __construct($name) {
		$this->name = $name;
	}
}

$vector = new Organisation('Вектор');

$dep1 = new Department('dep1');
$vector->addDepartment($dep1);
$vector->addDepartment($dep1);
$dep2 = new Department('dep2');
$vector->addDepartment($dep2);


var_dump($vector);