There is some smart object Processes Manager which i have created for my application. It can control the maximum of simultaneously running processes.

Proccesmanager class:
<?php 
class Processmanager {
    public $executable = "C:\\www\\_PHP5_2_10\\php";
    public $root = "C:\\www\\parallelprocesses\\";
    public $scripts = array();
    public $processesRunning = 0;
    public $processes = 3;
    public $running = array();
    public $sleep_time = 2;
    
    function addScript($script, $max_execution_time = 300) {
        $this->scripts[] = array("script_name" => $script,
                            "max_execution_time" => $max_execution_time);
    }
    
    function exec() {
        $i = 0;
        for(;;) {
        // Fill up the slots
        while (($this->processesRunning<$this->processes) and ($i<count($this->scripts))) {
        echo "<span style='color: orange;'>Adding script: ".$this->scripts[$i]["script_name"]."</span><br />";
        ob_flush();
        flush();
        $this->running[] =& new Process($this->executable, $this->root, $this->scripts[$i]["script_name"], $this->scripts[$i]["max_execution_time"]);
        $this->processesRunning++;
        $i++;
        }
        
        // Check if done
        if (($this->processesRunning==0) and ($i>=count($this->scripts))) {
            break;
        }
        // sleep, this duration depends on your script execution time, the longer execution time, the longer sleep time
      sleep($this->sleep_time);
      
      // check what is done
        foreach ($this->running as $key => $val) {
                if (!$val->isRunning() or $val->isOverExecuted()) {
            if (!$val->isRunning()) echo "<span style='color: green;'>Done: ".$val->script."</span><br />";
            else echo "<span style='color: red;'>Killed: ".$val->script."</span><br />";
                    proc_close($val->resource);
                    unset($this->running[$key]);
                    $this->processesRunning--;
            ob_flush();
            flush();
                }
            }
        }
    }
}
?>

Process class:
<?php
class Process {
    public $resource;
    public $pipes;
    public $script;
    public $max_execution_time;
    public $start_time;
    
    function __construct(&$executable, &$root, $script, $max_execution_time) {
        $this->script = $script;
        $this->max_execution_time = $max_execution_time;
        $descriptorspec    = array(
            0 => array('pipe', 'r'),
            1 => array('pipe', 'w'),
            2 => array('pipe', 'w')
        );
        $this->resource    = proc_open($executable." ".$root.$this->script, $descriptorspec, $this->pipes, null, $_ENV);
        $this->start_time = mktime();
    }
    
    // is still running?
    function isRunning() {
        $status = proc_get_status($this->resource);
        return $status["running"];
    }

    // long execution time, proccess is going to be killer
    function isOverExecuted() {
        if ($this->start_time+$this->max_execution_time<mktime()) return true;
        else return false;
    }

}
?>

Example of using:
<?php
$manager = new Processmanager();
$manager->executable = "C:\\www\\_PHP5_2_10\\php";
$manager->path = "C:\\www\\parallelprocesses\\";
$manager->processes = 3;
$manager->sleep_time = 2;
$manager->addScript("script1.php", 10);
$manager->addScript("script2.php");
$manager->addScript("script3.php");
$manager->addScript("script4.php");
$manager->addScript("script5.php");
$manager->addScript("script6.php");
$manager->exec();
?>

And possible output:

Adding script: script1.php
Adding script: script2.php
Adding script: script3.php
Done: script2.php
Adding script: script4.php
Killed: script1.php
Done: script3.php
Done: script4.php
Adding script: script5.php
Adding script: script6.php
Done: script5.php
Done: script6.php