Low memory usage collection

Collection.php

<?php

namespace jakefolio;

class Collection implements \ArrayAccess, \Countable, \IteratorAggregate
{
    protected $items;

    public function __construct($items = [])
    {
        if ($items instanceof \Traversable) {
            $this->items = $items;
        }

        if (is_array($items)) {
            $this->items = new \ArrayIterator($items);
        }
    }

    public function push($item)
    {
        $this->items->append($item);
    }

    public function map(callable $callback)
    {
        return new static(new MapIterator($this->items, $callback));
    }

    public function reduce(callable $callback, $initialValue = null)
    {
        $result = $initialValue;
        foreach ($this->items as $value) {
            $result = $callback($result, $value);
        }

        return $result;
    }

    public function has($item)
    {
        while ($this->items->valid()) {
            if ($this->items->current() === $item) {
                return true;
            }
            $this->items->next();
        }

        return false;
    }

    public function filter(callable $callback)
    {
        return new static(new \CallbackFilterIterator($this->items, $callback));
    }

    public function each(callable $callback)
    {
        foreach ($this->items as $key => $value) {
            $callback($value, $key);
        }
    }

    public function getIterator()
    {
        yield from $this->items;
    }

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

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

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

    public function offsetSet($key, $value)
    {
        if (is_null($key)) {
            $this->push($value);
        } else {
            $this->items[$key] = $value;
        }
    }

    public function offsetUnset($key)
    {
        unset($this->items[$key]);
    }
}

example.php

<?php

$collection = new jakefolio\Collection;
for ($i = 0; $i <= 10000; $i++) {
    $collection->push("John Doe {$i}");
}

$newCollection = $collection->map(function ($row) {
    return $row . ' YOLO';
})->map(function ($row) {
    return 'WAT ' . $row;
})->filter(function ($row) {
    return preg_match('/(WAT John Doe \d{3} YOLO)$/', $row);
})->each(function ($value) {
    echo $value . PHP_EOL;
});

MapIterator.php

<?php 

namespace jakefolio;

class MapIterator extends \IteratorIterator {

    protected $map;

    public function __construct(\Traversable $iterator, callable $callback)
    {
        parent::__construct($iterator);
        $this->map = $callback;
    }

    public function current()
    {
        return $this->map->__invoke(parent::current(), $this->key());
    }
}