Размер скидки minishop2

<?php
$price = empty($price) ? 0 : $price;
$old_price = empty($old_price) ? 0 : $old_price;

if($old_price > 0){
  return ''.round((($old_price - $price) / $old_price) * 100);
} 
return '';

<?php
return ($modx->resource->get('price') - $modx->resource->get('old_price'))/$modx->resource->get('price')*100;