ZipIterator: iterates over multiple traversables in parallel. Requirements: PHP 5.4

ZipIterator.php

class ZipIterator implements \Iterator
{
    const ZIP_ALL = '{6087F687-D674-4549-AFBF-8D4DC07FE06F}';

    private $_iterators;

    private $_zipAll = false;

    public function __construct()
    {
        foreach (func_get_args() as $argument) {
            if ($argument === self::ZIP_ALL) {
                $this->_zipAll = true;
            }
            else if (is_array($argument)) {
                $this->_iterators[] = new \ArrayIterator($argument);
            }
            else if ($argument instanceof \Traversable) {
                $this->_iterators[] = $argument;
            }
            else {
                throw new Exception("All arguments to ".__CLASS__." must be arrays or implement Traversable.");
            }
        }
    }

    public function current()
    {
        $result = [];
        foreach ($this->_iterators as $iterator) {
            $result[] = $iterator->current();
        }

        return $result;
    }

    public function next()
    {
        foreach ($this->_iterators as $iterator) {
            $iterator->next();
        }
    }

    public function key()
    {
        $result = [];
        foreach ($this->_iterators as $iterator) {
            $result[] = $iterator->key();
        }

        return $result;
    }

    public function valid()
    {
        foreach ($this->_iterators as $iterator) {
            if ($iterator->valid() ^ !$this->_zipAll) {
                return $this->_zipAll;
            }
        }

        return !$this->_zipAll;
    }

    public function rewind()
    {
        foreach ($this->_iterators as $iterator) {
            $iterator->rewind();
        }
    }
}