(WIP) Functions creating iterators for looping, based off the python itertools module.

iter.php

<?php
/**
 * Functions creating iterators for looping, based off the python itertools
 * module.
 *
 * @author Dan Horrigan <dan@dhorrigan.com>
 * @license MIT License
 * @copyright 2013 Daniel Horrigan
 */
namespace iter;

use InvalidArgumentException;

/**
 * A Generator which iterators through all the elements of all the arguments
 * in order until all items have been iterated over.
 *
 * Example:
 *
 *     foreach (chain(str_split('ABC'), str_split('DEF')) as $c) {
 *         echo $c." ";
 *     }
 *     // Output: A B C D E F
 * 
 * @return Iterator
 */
function chain()
{
    foreach (func_get_args() as $v) {
        if ( ! _is_traversable($v)) {
            throw new InvalidArgumentException("Value '{$v}', must be traversable.");
        }
        foreach ($v as $elem) {
            yield $elem;
        }
    }
}

/**
 * A simple counter Generator.  It will start at the given number and add `step`
 * to it infinitely.
 * 
 * @param  integer $start The number to start at.
 * @param  integer $step  The amount to increase by each iteration.
 * @return Iterator
 */
function counter($start = 0, $step = 1)
{
    $i = $start;
    while (true) {
        yield $i;
        $i += $step;
    }
}

/**
 * A generator that infinetly cycles through the given values.  You can send
 * either an array of values, or send the values as arguments.
 *
 * @param  mixed  $values If an array and the only argument, it is iterated over.
 * @return Iterator
 */
function cycle($values)
{
    if (func_num_args() > 1 || ! _acts_as_array($values)) {
        $values = func_get_args();
    }

    $i = 0;
    $len = count($values);
    while (true) {
        if ($i === $len) {
            $i = 0;
        }
        yield $values[$i];
        $i++;
    }
}

/**
 * The same as the PHP `range()` function, except implemented with an iterator.
 *
 * @param  integer  $start First value of the sequence.
 * @param  integer  $end   The value to end the sequence at.
 * @param  integer  $step  Amount to increment with each iteration.
 * @return Iterator
 */
function xrange($start, $end, $step = 1)
{
    foreach (counter($start, $step) as $i) {
        yield $i;
        if ($i === $end) {
            break;
        }
    }
}

/**
 * Repeats the given item either infinitly or the given amount of times.
 * 
 * @param  mixed    $item  The item to repeat.
 * @param  integer  $times The number of times to repeat (null for inf)
 * @return Iterator
 */
function repeat($item, $times = null)
{
    if (is_null($times)) {
        while (true) {
            yield $item;
        }
    } else {
        for ($i = 0; $i < $times; $i++) {
            yield $i;
        }
    }
}

/**
 * Checks if the given value can be traversed using a foreach loop.
 * 
 * @param  mixed  $value  The value to check.
 * @return boolean
 */
function _is_traversable($value)
{
    return is_array($value) || is_a($value, 'Traversable');
}

/**
 * Checks if the given value can be accessed like an array.
 * 
 * @param  mixed  $value  The value to check
 * @return boolean
 */
function _acts_as_array($value)
{
    return is_array($value) || (is_a($value, 'ArrayAccess') && is_a($value, 'Countable'));
}