运行PHP exec（）异步/多进程

I'm currently running these lines in a PHP script:

foreach ($list as $route)
{
    exec('php-cgi ./nextbus-route_stop_predictions.php route=' . $route['route_id']);
}

nextbus-route_stop_predictions.php takes about 15 seconds to run and exec() is ran about 12 times. I was wondering if PHP was able to run these those command lines without needing to wait for the output of the previous exec(). So basically, I want to run this asynchronously/multi-processes.

Update

For anyone still looking for the answer, I used nohup and piped the output to /dev/null

pcntl_fork() may be able to help if you're on *nix.

The pcntl_fork() function creates a child process that differs from the parent process only in its PID and PPID. Please see your system's fork(2) man page for specific details as to how fork works on your system.

If you are running php as an Apache module you can do the trick mentioned here

http://joseph.randomnetworks.com/archives/2005/10/21/fake-fork-in-php/

exec("php-cgi ./nextbus-route_stop_predictions.php route=" . $route['route_id'] . " 2>/dev/null >&- < &- >/dev/null &");

Basically rerouting stderr,stdout and sending the process into the background.