Linked list in PHP

Item.php

<?php

class Item {
    public $value, $key, $next = null;

    public function __construct($key, $value) {
        $this->key = $key;
        $this->value = $value;
    }

    public function next() {
        return $this->next;
    }
}

LinkedList.php

<?php
require_once 'Item.php';

class LinkedList {
    protected $items = array();
    public $first=null,$last=null;

    public function append($key, $value) {
        $item = new Item($key, $value);
        $this->items[] = $item;

        if ($this->last) $this->last->next = $item;
        $this->last = $item;

        if ($this->first === null) $this->first = $item;
        return $this;
    }

    public function prepend($key, $value) {
        $item = new Item($key, $value);
        array_unshift($this->items, $item);

        if ($this->first) $item->next = $this->first;
        $this->first = $item;

        if ($this->last === null) $this->last = $item;
        return $this;
    }
}

LinkedListTest.php

<?php
require_once 'LinkedList.php';

class LinkedListTest extends PHPUnit_Framework_TestCase {
    public function testLinkedList() {
        $list = new LinkedList;
        $data = array('foo','bar');
        foreach ($data as $k => $v) $list->append($k, $v);

        $this->assertEquals($data[0], $list->first->value);
        $this->assertEquals($data[1], $list->last->value);
        $this->assertNull($list->last->next());

        // Prepend data
        $prepend = array('a','b');
        foreach (array_reverse($prepend) as $k => $v) $list->prepend($k, $v);

        $item = $list->first;
        foreach (array_merge($prepend, $data) as $value) {
            $this->assertEquals($value, $item->value);
            $item = $item->next();
        }
    }
}