$_post复选框值未在echo中设置

如果复选框被选中，为什么我不能接收，如果我正在使用echo，为什么不能接收。如果需要，我可以添加代码的其他部分。感谢

HTML代码

                    <?php 
                    $k=1;
                    foreach($prods as $prd){
                        echo '<input type="text" style="visibility:hidden;display:none;" class="form-control" name="prodid" placeholder="name" value="'.$prd['id'].'">';
                        echo '<tr>
                        <td class="movable">
                            <img src=".//assets/img/images_19.png">
                        </td>
                        <td>
                            <input type="checkbox" name="prod_selected'.$prd['id'].'" value="checked">
                        </td>';  

PHP代码：这是新产品PHP，在这里我也尝试删除一个产品，通过选中它的复选框并点击删除按钮。

<?php 
require_once "../assets/class/database.class.php";
require_once "../assets/class/category.database.class.php";
require_once "../assets/class/picture.database.class.php";
require_once "../assets/class/manufacturer.database.class.php";
require_once "../assets/class/tag.database.class.php";

require_once "../assets/class/product.database.class.php";
require_once "../assets/class/product_category.database.class.php";
require_once "../assets/class/product_picture.database.class.php";
require_once "../assets/class/product_description.database.class.php";
require_once "../assets/class/product_tag.database.class.php";
require_once "../assets/class/product_related.database.class.php";
require_once "../assets/class/functions.php";
var_dump($_POST['test']);
var_dump($_POST['test']);
if ($_POST['submit']=="add") {
    $nameid=rand(10,500);
    $product->_addNewProduct("Newproduct".$nameid);
    $pid=$product->_getid("Newproduct".$nameid);
    $url="http://dev.codemyworld.com/sven/product?id=".$pid[0];
    header('Location:'.$url);
    }
    $check='prod_selected'.$_POST['prodid'];
if (($_POST['submit']=="delete")&&(isset($_POST['prod_selected']))){
    $product->_removeProd($_POST['prodid']);
}
var_dump($check);
var_dump($_POST['$check']);
?>