Async PHP v2 (This is another attempt at Async PHP using stream socket pair capabilities.

async-v2.php

<?php

declare(strict_types=1);

function async(Closure $task): Closure
{
    static $resolved = [];

    static $id = 0;

    $id++;

    if (! extension_loaded('pcntl') || ! extension_loaded('posix')) {
        return fn () => $task($id);
    }

    [$parent, $child] = stream_socket_pair(STREAM_PF_UNIX, STREAM_SOCK_STREAM, STREAM_IPPROTO_IP);

    $pid = pcntl_fork();

    if ($pid === 0) {
        fclose($parent);

        $result = $task($id);

        fwrite($child, serialize($result));

        fclose($child);

        exit(0);
    }

    return function () use ($pid, $id, $child, $parent, &$resolved) {
        if (isset($resolved[$id])) {
            return $resolved[$id];
        }

        fclose($child);

        pcntl_waitpid($pid, $status);

        $serialized = fgets($parent);

        fclose($parent);

        return $resolved[$id] ??= unserialize($serialized);
    };
}

function await(Closure|array $tasks): mixed
{
    if (! is_array($tasks)) {
        return $tasks();
    }

    return array_map(
        fn (Closure $task): mixed => $task(), $tasks,
    );
}

$start = microtime(true);

$hello = async(function (int $id) {
    echo "Running task {$id}\n";

    sleep(2);

    return "Hello from task {$id}";
});

$task2 = async(function (int $id) {
    echo "Running task {$id}\n";

    sleep(3);

    return "Hello from task {$id}";
});

var_dump(await([$hello, $task2]));

$end = microtime(true);

echo $end - $start;