Just FYI you have to make sure that you are allowed to execute system commands from your PHP scripts.
A lot of servers have this feature disabled.
However, if you want to turn this back on I believe you can do so in the php.ini file, you would have to turn safe_mode to off.
When allowing user-supplied data to be passed to this function, use escapeshellarg() or escapeshellcmd() to ensure that users cannot trick the system into executing arbitrary commands.
If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.
Note: When safe mode is enabled, you can only execute files within the safe_mode_exec_dir. For practical reasons, it is currently not allowed to have .. components in the path to the executable.