hi everyone

i want to ask about converting avi files to flv using FFMPEG - php
it works fine with some avi s and convert it to flv
but not all of the avi files
i tried to convert an avi file to flv , mp4 , mp3 , mpeg , dv , avi also !
and the output file size is 0 byets !

the command i used is :

$x=$ffmpeg." -i files/THIS2.avi files/THIS2.flv";
if (exec($x))
echo "done!";

it's print nothing and the output flv size is 0 bytes
i goggled another FFMPEG commands
but no results
all of them output a 0 bytes flv file


i need your help plz
thanks ..