I have always used:
$pid = exec(\"/usr/local/bin/php file.php $args > /dev/null & echo \\$!\");
But I am using an XP virtual machine to
You will have to install an extra extension, but found the solution located at Uniformserver's Wiki.
UPDATE
After some searching you might look into tasklist which coincidently, you may be able to use with the PHP exec command to get what you are after.
tasklist
exec