function add_gst( $_ni_cost_goods = null, $multiplier = 1.1) {
    if ( !empty( $_ni_cost_goods ) ) {
        // strip any extra characters from price
        $_ni_cost_goods = preg_replace("/[^0-9,.]/", "", $price);
        // perform calculations
        return $_ni_cost_goods * $multiplier; 
    }
}
  
   
    $
   
   在多个地方签署“NIU cost”商品。也就是说,你也在访问一个变量
   
    $price
   
   它不在函数的范围内。