Space after causing problem. Why?

Question

After a several hours looking for this bug, I was socked and a bit upset to realize it was due to a espace character after ?> tag in my php script.

The problem

I was simply trying to show a file that was storage in a BLOB table in a database using PHP. After investigating and isolating the problem, it seemed to be around these lines of code bellow.

problem.php

<?php
  require_once('empty-script.php');
  $data = file_get_contents('./images/download.jpeg');
  header("Content-Type: image/jpeg");
  echo $data;
?>

empty-script.php

<?php
?> 

The Bug

Instead of getting an output, this was the outcome on the browser. Not desired.

Getting Nuts

At some point I decided to comment the require_once. After noticing the bug was gone, I was about to blame it for the issue. Then it occurred me to create another empty file and import it.

The new empty file was working, require_once not necessarily was the one to be blamed. After a few minutes I noticed that there was an extra space after ?> in empty-script.php that was causing the image/file to not be rendered.

The Question

Is that a bug from PHP? Am I unaware of a documentation regarding this? Why this happens?

This is my server configuration