What permissions are needed to run a system() command within a php script that writes to a folder?
Solution 1:
A few random thoughts:
How long does your ffpmpeg command takes ? if it takes longer than the value of max_execution_time in php.ini, I believe that the command is canceled.
try to use only full paths for the ffmpeg binary and the input/output files. Although, if you have a zero size file outputted, it shouldn't be that.
Does apache have the permission to run the ffmpeg binary ?
try a basic command in your system call, such as "touch test.txt" just to check if the problem is from ffmpeg or your php script.