Apply a user supplied function to every member of an array, in parallel

array_walk_async.php

<?php

/**
 * Apply a user supplied function to every member of an array, in parallel.
 *
 * @param  array  $array
 * @param  callable  $callback First arg is the value, second arg is the key.
 * @param  mixed  $arg Will be passed as the third parameter to the callback.
 * @param  int  $maxProcesses
 * @return bool
 */
function array_walk_async(array $array, callable $callback, $arg = null, int $maxProcesses = 20): bool
{
    if (! function_exists('pcntl_fork')) {
        return array_walk($array, $callback, $arg);
    }

    $pids = [];

    foreach ((array) $array as $key => $value) {
        if (count($pids) >= $maxProcesses) {
            $pid = pcntl_waitpid(-1, $status);

            unset($pids[$pid]);
        }

        $pid = pcntl_fork();

        if ($pid > 0) {
            $pids[] = $pid;
        }

        if ($pid === 0) {
            $callback($value, $key, $arg);

            exit;
        }
    }

    foreach ($pids as $pid) {
        pcntl_waitpid($pid, $status);
    }

    return true;
}