Dynamic request list with Guzzle (see https://stackoverflow.com/questions/42754389/guzzle-pool-wait-for-requests/43525426#43525426 for details)

main.php

<?php

namespace GuzzleDynamicRequests;

require __DIR__ . '/vendor/autoload.php';

use GuzzleHttp\Client;
use GuzzleHttp\Psr7\Response;

// Do not extend IteratorIterator, because it cashes the return values somehow!
class MapIterator implements \Iterator
{
    /**
     * @var \Iterator
     */
    private $inner;
    private $handler;

    public function __construct(\Iterator $inner, callable $handler)
    {
        $this->inner = $inner;
        $this->handler = $handler;
    }

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

    public function current()
    {
        return call_user_func($this->handler, $this->inner->current(), $this->inner);
    }

    public function rewind()
    {
        $this->inner->rewind();
    }

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

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

// Do not extend IteratorIterator, because it cashes the return values somehow!
class ExpectingIterator implements \Iterator
{
    /**
     * @var \Iterator
     */
    private $inner;
    private $wasValid;

    public function __construct(\Iterator $inner)
    {
        $this->inner = $inner;
    }

    public function next()
    {
        if (!$this->wasValid && $this->valid()) {
            // Just do nothing, because the inner iterator has became valid.
        } else {
            $this->inner->next();
        }

        $this->wasValid = $this->valid();
    }

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

    public function rewind()
    {
        $this->inner->rewind();

        $this->wasValid = $this->valid();
    }

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

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

// Just for the example, to show the dynamic addition.
$next = new \SplDoublyLinkedList();
$next->push('http://yandex.ru');
$next->push('http://yahoo.com');
$next->push('http://rambler.com');
$next->push('http://bbc.co.uk');
$next->push('http://facebook.com');
$next->push('http://feedly.com');

$httpClient = new Client();

// MapIterator is just better for readability.
$generator = new MapIterator(
    // Initial data. This object will be always passed as the second parameter to the callback below
    new \ArrayIterator(['http://google.com']),
    function ($request, $array) use ($httpClient, $next) {
        return $httpClient->requestAsync('GET', $request)
            ->then(function (Response $response) use ($request, $array, $next) {
                // The status code for example.
                echo $request . ': ' . $response->getStatusCode() . PHP_EOL;

                // New requests.
                $array->append($next->shift());
                $array->append($next->shift());
            });
    }
);

// The "magic".
$generator = new ExpectingIterator($generator);

// And the concurrent runner.
$promise = \GuzzleHttp\Promise\each_limit($generator, 5);

$promise->wait();