I just noticed a typo in one line of my code above. I left out a closing ) at the end of this line:

<h2 class="middleBoxHeading" id="middlebox<?php echo $i; ?>Heading"><?php echo constant('BOX_HEADING_MIDDLEBOX' . $i); ?></h2>

The lack of that would cause a fatal error, and adding it may fix the problem.