Watermark mp4/m4v in PHP

Question

Is there a way to watermark mp4/m4v files in PHP? The files are already saved on the server, but when displaying, I want a watermark to be put over through PHP (header-output).

Here is my current PHP code:

$fp = fopen($path, 'rb');

// Change the header to whatever your file's mime type is
header("Content-Type: " . $ctype);
header("Content-Length: " . filesize($path));

fpassthru($fp);
exit;