<?php
include("conn.php");
session_start();
$a=$_SESSION['ddh'];
$b=$_POST['mend'];
$xg="update dd set ssl='".$b."' where ddh='".$a."'";
mysql_query($xg) or die("错误");
if($xg==true)
    {
echo "<script>alert('修改成功');location.href='indexs.php?lmbs=3';</script>";
}
  else
     {
 echo "<script>alert('修改失败');location.href='indexs.php?lmbs=3';</script>";
 }?>
运行后,显示错误,请问错误在哪?谢谢