I am a fucking idio. Anyway, it's fixed now.

threadr/board/board.php

@@ -15,7 +15,7 @@ foreach($statement->fetchAll() as $ROW) {
 
   // add line breaks to post content, to be replaced with proper makrdown support in the future (see #44)
   $newlines = array("\r\n", "\n\r", "\r", "\n"); // two-character newlines first to prevent placing two line breaks instead of one
-  str_replace("\n", "<br />", $post_content);
+  $post_content = str_replace($newlines, "<br />", $post_content);
 
   echo "<section>
 <h1>$post_title</h1>