<?php

class Organisation {

	private $name;
	private $departments = [];

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

	public function addDepartment(Department $dep) {

		$exist = false;
		
		foreach ($this->$departments as $department) {
			if ($department === $dep) {
				$exist = true;
			}
		}

		if ($exist == false) {
			$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);

var_dump($vector);