ffmpeg is the tool of choice for this. It's CPU-intensive because that's the nature of encoding video or audio.
You can simply rip the stream (assuming original mp3 audio is in the stream) from the track and save it. I've used this
bash snippet before for directories of
for i in *.flv;
do ffmpeg -i "$i" -acodec copy `basename $i .flv`-`date +%H%M%S%N`.mp3;
If the stream is not natively in mp3 format, you need to re-encode. Or perhaps you want a different bitrate, etc. This task will definitely consume more CPU than the former:
ffmpeg -i "$file" -f mp3 -vn -acodec libmp3lame -ab 192 `basename $file .flv`-`date +%H%M%S%N`.mp3;
edit: to limit to one core:
taskset 1 ffmpeg <rest of args> --
taskset is part of the
util-linux package on Debian systems. You may also want to
renice the process, setting its priority value to something in the positive range (sounds backwards, but lower
nice value = more cpu time). As always,
This is how you do what you want to do with ffmpeg from the shell -- implementing in PHP is left as an exercise for the reader.