Codeigniter表单验证回调函数抛出错误消息

I have two input field carrying Product Price and Product Strike Price. In which Product Strike Price can be zero but cannot be less than or equal to product price. I created a callback function but it does not work it throws error as 'Unable to access an error message corresponding to your field name Product Strike Price.(price_check)'

Here is Callback function:

function price_check(){
                $pd_price = intval($this->input->post('product_price'));
                $pd_strikeprice = intval($this->input->post('product_strike_price'));

                if($pd_strike_price > $pd_price OR $pd_strike_price = 0){
                    return true;
                }else{

                    $this->form_validation->set_message('price_check', 'Product Strike Price can be zero(0) but cannot be less than or equal to Product Price.');
                return false;
                }
            }

And here is the Form validation:

$this->form_validation->set_rules('product_strike_price', 'Product Strike Price', 'trim|required|is_natural|callback_price_check');

Some one please help me in resolving the issue.

pass the the input product_price to your call back function like this.

callback_price_check['.$this->input->post('product_price').'] 

by this, it means you pass two arguments to your callback function so it would be like this in the next step.

 public function price_check($pd_strikeprice, $pd_price){
         if($pd_strike_price > $pd_price || $pd_strike_price = 0){
                 return true;
         }else{

                 $this->form_validation->set_message('price_check', 'Product Strike Price can be zero(0) but cannot be less than or equal to Product Price.');
                 return false;
             }
    }

Please let me know the result.