我是Joomla社区的新手。在evinessmart的产品类别中，我设置了排序字段，如按字母顺序对产品，新旧产品进行排序。我在default.php中编写了php代码。但是下拉菜单在网页上不可见。下面是PHP的代码。

<span class="category-order-by" id="custom-category-order-by">
                    <div class="form-group">
                      <label for="custom_orderby"><?php echo vmText::_ ('COM_VIRTUEMART_ORDERBY') ?></label>
                      <select class="form-control select-picker" id="custom_orderby" name="custom_orderby" onchange="document.location.replace(this.value);">
                        <option disabled selected>Select an option</option> 
                        <option <?php if(strpos($url,'/by,product_sales?keyword=') !== false) {?> selected <?php } ?> value="<?php echo $urlPath ?>/by,product_sales?keyword=">Best Selling</option>
                      </select>
                    </div>
                </span>