PHP Undefined Constant PHP_ROUND_HALF_DOWN

The mode argument was only added in PHP 5.3.0. If you're running an earlier version of PHP, then the mode option constants (PHP_ROUND_HALF_UP, PHP_ROUND_HALF_DOWN, PHP_ROUND_HALF_EVEN, and PHP_ROUND_HALF_ODD) won't be defined

EDIT

You can't use the mode argument for round() prior to 5.3.0, but you can achieve the equivalent by combining functions:

round(floor($value * 100) / 100,2); //  to round down to 2dp
round(floor($value * 1000) / 1000,3); //  to round down to 3dp
round(ceil($value * 100) / 100,2); //  to round up to 2dp

The rounding mode was added in PHP 5.3. Make sure you're running at least that version.

You can see which version you're running by placing the following in a PHP file:

var_dump(phpversion());

PHP_ROUND_HALF_DOWN requires PHP 5.3.0 as seen here: http://php.net/manual/en/math.constants.php

You're probably on a lower PHP version.