mailgun-php/Mailgun/Guzzle/Iterator/MapIterator.php

<?php

namespace Guzzle\Iterator;

use Guzzle\Common\Exception\InvalidArgumentException;

/**
 * Maps values before yielding
 */
class MapIterator extends \IteratorIterator
{
    /** @var mixed Callback */
    protected $callback;

    /**
     * @param \Traversable   $iterator Traversable iterator
     * @param array|\Closure $callback Callback used for iterating
     *
     * @throws InvalidArgumentException if the callback if not callable
     */
    public function __construct(\Traversable $iterator, $callback)
    {
        parent::__construct($iterator);
        if (!is_callable($callback)) {
            throw new InvalidArgumentException('The callback must be callable');
        }
        $this->callback = $callback;
    }

    public function current()
    {
        return call_user_func($this->callback, parent::current());
    }
}