php批量删除操作

<form action="piliangshanchu.php" method="post" >
<table border="1" cellspacing="0" cellpadding="0">
  <tr>

    <td width="200">
      <input type="checkbox" value="''" name="dx" onclick="checkall(this)" />
      编号</td>
    <td width="200">姓名</td>
    <td width="200">电话</td>
    <td width="200" >分组</td>
    <td width="200" >操作</td>
  </tr>
<tr>
    <td>
    <input type='checkbox' value='{$attr[0]}' name='item[]' class='ck' />
    {$attr[0]}</td> 

    <td>{$str}</td>
    <td>{$attr[2]}</td>
    <td>{$nation}</td>

</tr>


</table>

  <input type="submit" value="批量删除"/>
  </form>

<script>
  function xxx(qx)
  {

//全选多选的选中状态
    var ck = document.getElementsByClassName("ck");
 //让下面所有的多选选中状态改变
    if(qx.checked)
    {
      for(i = 0;i < ck.length ; i++)
      {
        ck[i].setAttribute("checked","checked");
//状态改变为选中
      }
    }
    else
    {
      for(var i = 0;i < ck.length;i++)
      {
        ck[i].removeAttribute("checked");
//移除选中
      }
    }
  }
</script>

<?php
$arr = $_POST["item"];
$db = new mysqli("localhost","root","12345678","heiheihei");
//foreach($arr as $v)
//{
//  $sql = "delete from contacts WHERE id='{$v}'";
//  $db->query($sql);
//}
$str = implode("','",$arr);//拼接字符，
$sql = "delete from contacts WHERE id in('{$str}')";
//2','8','4
if($db->query($sql))//判断是否查询成功，
{
  header("location:shouye.php");
  //成功就跳转
}



?>