Let's try another thing:
<?php
global $txt;
require_once('SSI.php');
loadLanguage('Post');
echo 'first: ' . sprintf($txt['attach_restrict_attachmentPostLimit'], comma_format($modSettings['attachmentPostLimit'], 0)) . '<br>';
echo 'second: ' . sprintf($txt['attach_restrict_attachmentPostLimit'], comma_format(1024, 0)) . '<br>';
echo 'third: ' . sprintf($txt['attach_restrict_attachmentPostLimit'], '1024') . '<br>';
put this code into a file (e.g test.php), then upload it to your forum directory (where is SSI.php), and finally reach the file from the browser, what do you see?