This works fine for me. Remember that (1) the "\n" escape character is a literal "n" within a single-quoted string literal, not a line break, and (2) in HTML output, linebreaks within normal text are treated as word spaces, so you either need to use a tag/CSS style that treats newlines a line breaks, or replace them with <br> tags:
<?php
define('LINE_BREAK', "this is\na test");
printf("<pre>%s</pre>", LINE_BREAK);
// or...
echo nl2br(LINE_BREAK);
?>