shell_exec echo'ing too fast or too late

Question

So I read throug this article: https://trac.ffmpeg.org/wiki/PHP and found this:

Answer 1

You might want to use flush before you do your shell_exec. There are a few caveats to using this which are well explained in the php documentation page. Hope this helps.

<?php
  echo "Starting ffmpeg...\n\n";

  ob_flush();
  flush();
  echo shell_exec("ffmpeg -i input.avi output.avi >/dev/null 2>/dev/null");

  // a sleep could effectively be the same as your shell_exec operation
  // sleep(5);

  echo "Done.\n";
  ob_end_flush();