Collection class in PHP

simple-collection.php

<?php

/**
 *   @title: Simple collection classe in PHP
 *   @link : http://www.sitepoint.com/collection-classes-in-php/
 */

class Collection 
{
    private $items = array();
 
    public function addItem($obj, $key = null) 
    {
        if ($key == null) 
        {
            $this->items[] = $obj;
        }
        else 
        {
            if (isset($this->items[$key])) 
            {
                throw new KeyHasUseException("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.");
        }
    }

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

    public function keys() 
    {
        return array_keys($this->items);
    }

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

    public function keyExists($key) 
    {
        return isset($this->items[$key]);
    }

}

/**
 * @usage : 
 */

class Salut
{
    private $name;
    private $number;

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

    public function __toString() 
    {
        return $this->name . " is number " . $this->number;
    }
}

$c = new Collection();
$c->addItem(new Salut("Steve", 14), "steve");
$c->addItem(new Salut("Ed", 37), "ed");
$c->addItem(new Salut("Bob", 49), "bob");

$c->deleteItem("steve");

try 
{
    $c->getItem("steve");
}
catch (KeyInvalidException $e)
{
    print "The collection doesn't contain Steve.";
}