php在Windows中执行exec时挂起

I am converting a video using ffmpeg from php in Windows using following code

<?php

exec('ffmpeg -i input.mp4 -ar 22050 -ab 32k -r 25 -s 480x360 -vcodec h264 -qscale 2.5      output.flv');

?>

While converting script hangs until conversion completed.

What can i do about it? So that it can run in background

If your system is Linux I would just background the task by adding ' &' to the end of the command.

exec('ffmpeg -i input.mp4 -ar 22050 -ab 32k -r 25 -s 480x360 -vcodec h264 -qscale 2.5 output.flv &');

Or to be even more thorough (i.e. even if apache dies the file will still be transcoded):

exec('bash -c "exec nohup setsid ffmpeg -i input.mp4 -ar 22050 -ab 32k -r 25 -s 480x360 -vcodec h264 -qscale 2.5 output.flv > /dev/null 2>&1 &"')

Don't you have to call ffmpeg.exe on windows?