Custom Collection class for php

Collection.php

<?php

namespace App;


/**
 * Created by PhpStorm.
 * User: Vassilis Pallas
 * Date: 7/6/16
 * Time: 1:44 AM
 */
class Collection
{
    private $items = array();

    public function addItem($key = null, $obj)
    {
        if ($key == null) {
            $this->items[] = $obj;
        } else {
            if (isset($this->items[$key])) {
                // throw new KeyHasUseException("Key $key already in use.");
                throw new Exception("Key $key already in use.");
            } else {
                $this->items[$key] = $obj;
            }
        }
    }

    public function deleteItem($key)
    {
        if (isset($this->items[$key])) {
            unset($this->items[$key]);
        } else {
            // throw new KeyInvalidException("Invalid key $key.");
            throw new Exception("Invalid key $key.");
        }
    }

    public function getItem($key)
    {
        if (isset($this->items[$key])) {
            return $this->items[$key];
        } else {
            // throw new KeyInvalidException("Invalid key $key.");
            throw new Exception("Invalid key $key.");
        }
    }

    public function size()
    {
        return count($this->items);
    }
}