app\models\Invoice::isChanged PHP Method

    public function isChanged()
    {
        if ($this->getRawAdjustment() != 0) {
            return true;
        }
        foreach (['invoice_number', 'po_number', 'invoice_date', 'due_date', 'terms', 'public_notes', 'invoice_footer', 'partial'] as $field) {
            if ($this->{$field} != $this->getOriginal($field)) {
                return true;
            }
        }
        return false;
    }