Apply a user supplied function to every member of an array, in parallel. Similar to PHP's array_walk(), except the input array isn't passed by reference and the callbacks are ran asynchronously. The technique is highly portable and requires only PHP 5.4 and the PCNTL extension.

async-array-walk.php

<?php

/**
 * Apply a user supplied function to every member of an array, in parallel
 *
 * Similar to PHP's array_walk(), except the input array isn't passed by
 * reference and the callbacks are ran asynchronously. The technique is highly
 * portable and requires only PHP 5.4 and the PCNTL extension.
 *
 * The most obvious limitation is that we can't pass the input array by
 * reference to modify it, this is because child processes have no awareness of
 * the parent and therefore can't pass data back to the original process.
 *
 * @author Frankie Jarrett
 *
 * @param array    $array        The input array.
 * @param callable $callback     Takes on two parameters, the array parameter's
 *                               value being the first, and the key second.
 * @param int      $max_children Limit the maximum number of children (forks)
 *                               that can run in parallel. Defaults to 20.
 *
 * @return true
 */
function async_array_walk(array $array, callable $callback, $max_children = 20)
{
    $pids = [];

    foreach ($array as $key => $value) {

        // Sync (slow).
        if (! function_exists('pcntl_fork')) {
            $callback($value, $key);
        }

        // Async (fast).
        if (count($pids) >= (int) $max_children) {
            $pid = pcntl_waitpid(-1, $status);
            unset($pids[ $pid ]);
        }

        $pid = pcntl_fork();

        // Parent process.
        if ($pid > 0) {
            $pids[] = $pid;
        }

        // Child process (fork).
        if (0 === $pid) {
            $callback($value, $key);
            exit;
        }
    }

    // Wait for all forks to finish.
    foreach ($pids as $pid) {
        pcntl_waitpid($pid, $status);
    }

    return true;
}

example.php

<?php

$fruits = [
    'apples.txt' => 'I like apples',
    'bananas.txt' => 'I like bananas',
    'oranges.txt' => 'I like oranges',
];

async_array_walk($fruits, function ($value, $key) {
    sleep(2);
    file_put_contents($key, $value);
});

// All 3 files will be written in 2 seconds, not 6 seconds.