Typed collection in php (collection of objects) for usage in DDD projects.

TypedCollection.php

<?php

interface Vehicle{}
class Car implements Vehicle{}
class Track implements Vehicle{}
class Fruit{}

class Collection
{
    protected $items = [];
    protected $type;
    public function __construct($type)
    {
        $this->type = $type;
    }
    public function add($item)
    {
        if ($item instanceof $this->type) {
            if (!in_array($item, $this->items)) {
                $this->items[] = $item;
            }
        } else {
            throw new Exception(get_class($item) . ' must be instance of Vehicle');
        }
    }
    public function getItems()
    {
        if (count($this->items) != 0) {
            return $this->items;
        } else {
            throw new Exception('There is no items in collection');
        }
    }
    public function flush()
    {
        $this->items = [];
    }
    public function remove($item)
    {
        if (count($this->items) > 0) {
            foreach ($this->items as $key => $val) {
                if (get_class($item) == get_class($val)) {
                    unset($this->items[$key]);
                }
            }
        } else {
            throw new Exception('There is no items in collection');
        }
    }
    public function count()
    {
        return count($this->items);
    }
}
class VehicleCollection extends Collection
{
}
$vc = new VehicleCollection('Vehicle');
$car = new Car;
$track = new Track;
$fruit = new Fruit;
//$vc->add($fruit);
$vc->add($track);
$vc->add($car);
$vc->remove($track);
//$vc->flush();
echo $vc->count() . " items in collection \n";
$items = $vc->getItems();
foreach ($items as $key => $val) {
    echo $key . " => " . get_class($val) . "\n";
}