Hey guys I just wanted to share this solution that I found for the error: Cannot modify header information - headers already sent.
Let's say one starts out with a code that contains headers like this:
<?php
header('Content-disposition: attachment; filename="video"');
header('Content-type: video/mp4');
$video = $_POST['$video'] ;
readfile("$video");
?>
For some reason PHP doesn't like this. To see how to fix this view answer below.