Помогите посчитать налог - [VM1] Управление товарами, атрибутами, типами, ценами

$html = '';
		if( sizeof( $details) > 1 ) {
			$html .= '<br />'.$VM_LANG->_('VM_TAXDETAILS_LABEL').':<br />';
			
			foreach ($details as $rate => $value ) {
				if( !$auth['show_price_including_tax']) {
					$value /= $discount_factor;
				}
				$rate = str_replace( '-', $CURRENCY_DISPLAY->decimal, $rate )*100;
				$html .= $CURRENCY_DISPLAY->getFullValue( $value, 5, $currency ).' ('.$rate.'% '.$VM_LANG->_('PHPSHOP_CART_TAX').')<br />';
			}
		}
		return $html;

function approx( $value, $dec = 2 ) {
		$value += 0.0;
		$unit  = floor( $value * pow( 10, $dec + 1 ) ) / 10;
		$round = round( $unit );
		return $round / pow( 10, $dec );