FOR WINDOWS USERS
i had a look to start command. it's really usefull in fact.
this is how i launch a specific Apache SSL server
<?php
$cmd = "start /D C:\OpenSA\Apache /B Apache.exe -D SSL";
exec($cmd,$output,$rv);
?>
/D specify the path where application will be launched (equivalent to cd to path)
/B launch application without a console window
so to know the exact parameters of start, you could use
<?php
exec("start /? > start.txt");
?>
and you will get the help of start in a file named 'start.txt' in the same place you run the script
for WIN 98 you may need to modify a little your command
exec("COMMAND.COM /C START program args >NUL");
(taken from another post)