使用内置发票设置更改税款。如果你坚持硬编码的话。你可以这样做:
<?php
$customTaxRate_1 = 15;
$customTaxRate_2 = 25; //Norwegian sales tax phew :( But free health care :)
$this->db->select('(SELECT SUM('.$this->db->dbprefix('invoice_items').'.amount * '.$this->db->dbprefix('invoice_items').'.quantity * ('.$this->db->dbprefix('invoices').'.'.$customTaxRate_1.'/100 * '.$this->db->dbprefix('invoice_items').'.taxable)) FROM '.$this->db->dbprefix('invoice_items').' WHERE '.$this->db->dbprefix('invoice_items').'.invoice_id=' . $invoice_id . ') AS total_tax1', FALSE);
$this->db->select('(SELECT SUM('.$this->db->dbprefix('invoice_items').'.amount * '.$this->db->dbprefix('invoice_items').'.quantity * ('.$this->db->dbprefix('invoices').'.'.$customTaxRate_2.'/100 * '.$this->db->dbprefix('invoice_items').'.taxable)) FROM '.$this->db->dbprefix('invoice_items').' WHERE '.$this->db->dbprefix('invoice_items').'.invoice_id=' . $invoice_id . ') AS total_tax2', FALSE);
$this->db->select('(SELECT SUM('.$this->db->dbprefix('invoice_items').'.amount * '.$this->db->dbprefix('invoice_items').'.quantity + ROUND(('.$this->db->dbprefix('invoice_items').'.amount * '.$this->db->dbprefix('invoice_items').'.quantity * ('.$this->db->dbprefix('invoices').'.'.$customTaxRate_1.'/100 + '.$this->db->dbprefix('invoices').'.'.$customTaxRate_2.'/100) * '.$this->db->dbprefix('invoice_items').'.taxable), 2)) FROM '.$this->db->dbprefix('invoice_items').' WHERE '.$this->db->dbprefix('invoice_items').'.invoice_id=' . $invoice_id . ') AS total_with_tax', FALSE);
?>
请确保测试正确。我不熟悉这个软件。我的答案是基于你提供的代码。