For a recent project I needed to process some code in parallel. A colleague pointed me to the pcntl_fork() function, and it turns out it’s not quite as scary and complicated as I thought. So I turned the simple use case into a generic function. You use it something like this:

<?php
// A function to run
$makeDir = function($a) {
    shell_exec('mkdir '.shellescapearg($a));
}
 
// An array to process
$dirnames = array('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k');
 
// Run the process in parallel.
processParallel($makeDir, $dirnames, 8);

The actual function is below:

<?php
/**
 * Process in Parallel.
 *
 * Run a function (with no return result) on each item in an array in parallel.
 * Note: This function is only useful if order is not important, and you don't
 * need any return values from the function (i.e. no inter-process communication).
 *
 * @param mixed   $func  A closure function to apply to each item in parallel.
 * @param array   $arr   The array to apply function to.
 * @param integer $procs Number of processes to run in parallel.
 *
 * @return void
 */
function processParallel($func, array $arr, $procs=4)
{
    // Break array up into $procs chunks.
    $chunks   = array_chunk($arr, ceil((count($arr) / $procs)));
    $pid      = -1;
    $children = array();
    foreach ($chunks as $items) {
        $pid = pcntl_fork();
        if ($pid === -1) {
             die('could not fork');
        } else if ($pid === 0) {
            // We are the child process. Pass a chunk of items to process.
            array_walk($items, $func);
            exit(0);
        } else {
            // We are the parent.
            $children[] = $pid;
        }
    }
 
    // Wait for children to finish.
    foreach ($children as $pid) {
        // We are still the parent.
        pcntl_waitpid($pid, $status);
    }
}