echo exec("/usr/local/bin/ffmpeg  -i test.mp3  -codec:a libmp3lame -b:a 128k out.mp3");

$ffmpegpath= '/usr/local/bin/ffmpeg';
$file='test.mp3';
$exec_string = '"'.$ffmpegpath.'" -i "'.$file.'" -codec:a libmp3lame -b:a 128k out.mp3';
exec($exec_string);