As a note. I've tried the system() recommended on php.net/manual
In my case webstart.cmd starts a second .bat file (which actually calls a 3rd)
From WIDOWS to start multilpe php scripts from a parent script and do not wait for child scripts to finish, from the parent.php creat a X.bat file that contains all the commands to start child objects, something like:
start /B C:/[path_to_php]/php -f C:/child1.php > C:/[path_to_log1].txt
start /B C:/[path_to_php]/php -f C:/child2.php > C:/[path_to_log2].txt
and then run the X.bat width
<?php
system("CMD /C X.bat");
?>