Here is an interesting script I wrote.  It demonstrates how pcntl_fork() might be used as a useful tool.

<?php
/* This script serves the purpose of testing an algorithm designed to:
a.) Compare password hashes, or try passwords efficiently where the time to try a single password is 10 seconds.  
b.) Spawn threads to work simultaneously on comparing hashes.
c.) Restrict the number of threads open at a time.  
*/
//checks for divisibility
function divby($num,$den) {
    $result = $num/$den;
    $result2 = floor($result);
    if ($result == $result2) {
        return true;
        }
    else {
        return false; 
        }
    }
//checks whether a period of time fits into 2 second intervals occuring every 10 seconds.  Interval may increase or decrease in size to use more or less memory.  
function goodTime($elapsed) {
    $num = floor($elapsed);
    $num = $num/12;
    $min = floor($num);
    $min = 12*$min;
    $max = $min+2;
    if ($elapsed >= $min && $elapsed <= $max) {
        return "yes";
        }
    else {
        return "no";
        }
    }

$x = 30; //number of child threads
$pid = 1; //needed to create first thread
$xpass = md5('29');//hash to crack
$time = time();
$i = 1;
//parent spawns $x children.
while ($i <= $x) {
    if (file_exists('childcall.txt')) {
        unlink('childcall.txt');
        exit;
        }
    $elapsed = time()-$time;
    //children are only spawned during intervals occuring every 10 seconds leaving enough time for the previous batch of children to finish their task.
    if (goodTime($elapsed)=="yes") {    
        //Are we the parent?
        if ($pid != 0) {
            //Give birth to a child.  
            $pid = pcntl_fork();
            //create a record of how many children have been birthed.
            $arr[$i] = $i;
            $time2 = $elapsed;
            }
        //escort children out of the loop.
        if ($pid == 0) {
            $i = $x+1;
            }
        $i++;
        }
    }
//parent waits for children to finish playing. 
if ($pid) {
    $value = 1;
    while (!file_exists('childcall.txt')) {
        //wait
        }
    unlink('childcall.txt');
    $time = time()+2;
    while (time()<$time) {
        //wait
        }
    exit;
    }
//children take turns finding the highest array value, and changing it to 0
rsort($arr);
$value = max($arr);
$arr[$value] = 0;
$time = time()+10;
//simulate delay
while (time() < $time) {
    //wait
    }
//compare the high array value hash to the hash we are looking to crack.
if (md5($value) == $xpass) {
    echo "$value \n";
    }
if ($value == $x || md5($value) == $xpass) {
    $file = "childcall.txt";
    $content = true;
    file_put_contents($file,$contents);
    }
?>