hash-many.php

<?php

include dirname(__DIR__) . "/vendor/autoload.php";

use Amp\Process\Process;
use function Amp\Promise\all;

const COUNT = 8;

$passwords = [];

for ($i = 0; $i < COUNT; $i++) {
    $passwords[] = \random_bytes(20);
}

$start = microtime(true);
foreach ($passwords as $password) {
    \password_hash($password, PASSWORD_DEFAULT);
}
$syncTime = microtime(true) - $start;

function get_hash(string $password): \Generator
{
    /** @var Process $process */
    $process = yield Process::start("php hash.php");

    $stdIn = $process->getStdin();
    $stdIn->write($password);
    $stdIn->close();

    $result = '';
    $stream = $process->getStdout();
    while ($chunk = yield $stream->read()) {
        $result .= $chunk;
    }

    yield $process->join();

    return $result;
}

$asyncTime = 0;
Amp\Loop::run(function() use($passwords, &$asyncTime) {
    $promises = [];

    foreach ($passwords as $i => $password) {
        $promises[] = new \Amp\Coroutine(get_hash($password));
    }

    $start = microtime(true);
    yield all($promises);
    $asyncTime = microtime(true) - $start;
});

?>
Hashing <?= COUNT ?> passwords synchronously took <?= $syncTime ?> seconds
Hashing <?= COUNT ?> passwords asynchronously took <?= $asyncTime ?> seconds
Async took <?= ($asyncTime / $syncTime) * 100 ?>% of the time that sync took

hash.php

<?php echo \password_hash(\stream_get_contents(STDIN), PASSWORD_DEFAULT);