title: ffmpeg creates empty file
body: <p>I'm trying to convert an mp4 video into an mp3 audio file using ffmpeg and PHP. I have the ffmpeg shared library installed in <code>C:/wamp/www/ffmpeg</code>.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>$cmd = 'C:/wamp/www/ffmpeg/bin/ffmpeg.exe -i C:/wamp/www/blog/test.mp4 -vn -acodec copy C:/wamp/www/blog/test.mp3';
  exec($cmd, $output);
  </code></pre>
  
  <p>The problem is that while it creates <code>test.mp3</code>, it is a completely empty file. Also, <code>$output</code> is empty as well.</p>
  
  <p>Any help would be greatly appreciated.</p>