如何在从属下拉列表中选择多个选项？

Question

我要做的就是在依赖下拉列表中选择多个选项。以下是我的区域和亚松动的示例下拉代码。如何在下拉菜单和从属下拉菜单中选择多个选项？

    <select class="form-control" id="region_country_id" name="region_country_id">
              <?php foreach ($countries as $key => $value) { ?>
                                 <option value="<?php echo $value->id; ?>" 
                                    <?php if($product->region_country_id == $value->id){ echo "selected";} 
                                    ?>>
                                    <?php echo $value->name;?>

                                    </option>
                                     <?php } ?>


                                 </select>
                                <select class="form-control" id="region_id" name="region_id" style="margin-top: 10px;">
                                   <?php if(Session::get('branch_access') != 1){?>
                                      <option value="">All region</option>
                                            <?php } ?>
                                     <?php foreach ($regions as $key => $value)  { ?>
                                     <option value="<?php echo $value->id; ?>" <?php if($value->id == $product->region_id) { echo "selected";} ?>><?php echo $value->region; ?></option>
                                            <?php  } ?>
                                  </select>