新手在此请教各位大大checkbox多选delete问题。

if(isset($_POST['submit'])){
  if('Delete=Delete Selected'){
  mysql_query("DELETE FROM Persons WHERE Gid='$row[GuestId]'");
  }
}

 <tr>
  <td><?php echo ++$no+$page*5-5 . "." . "<input type='checkbox' name='Gid[]' value='$row[GuestId]'; />"?></td>
  <td><?php echo "Name : " . $row['Name'] . 
"<br>Email : " . $row['Email'] . 
"<br>Country : " . $row['Country'] . 
"<br><br>" . $row['Comment']?></td>
  <td><a href=''>[Edit] </a></td>
 </tr>
<?php
}
while($row = mysql_fetch_array($result));
echo "</table>";
}
我从中抽出的代码。重要的是要问，我在input的checkbox要获取选中的id并没有问题。但是要delete，传到上面的判断是就读不出来了。就连echo都读不到，我少了什么？不会是计算checkbox所选中的项吧？

$t = join(',', $_POST['Gid']);
$sql = "delete from Persons where gid in ($t)";