example.php

<?php
declare(strict_types=1);

$loop = new Pool();

$loop->add(function() {
    echo yield function() {
        return 1;
    };

    echo yield function() {
        return 3;
    };
});

$loop->add(function() {
    echo yield function() {
        return 2;
    };

    echo yield function() {
        return 4;
    };
});

$loop->run();

Pool.php

<?php
declare(strict_types=1);

class Pool
{
    /**
     * @var \Generator[]|array[]
     */
    private $iterators = [];

    /**
     * @param Closure $callback
     * @return Pool
     */
    public function add(\Closure $callback): Pool
    {
        /** @var \Generator $iterator */
        $this->iterators[] = $callback();

        return $this;
    }

    /**
     * @return iterable|\Generator[]
     */
    private function parallel(): iterable
    {
        while (\count($this->iterators)) {
            foreach ($this->iterators as $i => $iterator) {
                if (! $iterator->valid()) {
                    unset($this->iterators[$i]);
                    continue;
                }

                yield $iterator => $iterator->current();
            }
        }
    }

    /**
     * @return void
     */
    public function run(): void
    {
        foreach ($this->parallel() as $ctx => $value) {
            if ($value instanceof \Closure) {
                $ctx->send($value());
                continue;
            }

            $ctx->next();
        }
    }
}