Run external program in background

[eluser]eroy4u[/eluser]
In my web site, it allow people to upload an video and then call an external program to encode the video to flash video format.
I use exec('...') to open the external program for encoding.
However, encoding takes much time so my php script runs maybe half an hour.

Is there a way I can have my php script end quickly and run this external program in background?

Thanks if anyone can help.