php修改数据库数据不成功怎么办

<?php 
header('Content-Type:textml;charset=utf-8'); $username='游客'; 
//获取用户输入的账号和密码并连接数据库 
$account=$_POST['account']; $password=$_POST['password']; 
$db=new mysqli('localhost','root','12345678','newsmanage'); 
//查询数据库是否已经有相同账号 
$query1=$db->prepare("SELECT account FROM user"); 
$query1->bind_result($theaccount); $query1->execute(); 
while($query1->fetch()) { if ($account == $theaccount) { echo '<script>alert("该账号已被注册！");
location.href="register.html"</script>'; exit(); } } 
//将用户输入的账号和密码写入数据库 
$add=$db->prepare("INSERT INTO user VALUES (null,'general',null,?,?)"); 
$add->bind_param('ss',$account, $password); $add->execute(); 
//数据库对添加的新用户自动生成唯一ID，查寻此ID并赋值变量 
$query2=$db->prepare("SELECT userid FROM user WHERE account='$account'"); 
$query2->bind_result($userid); $query2->execute(); $query2->fetch(); 
//用username变量修改数据表中的数据 
$edit=$db->prepare("UPDATE user SET username=? WHERE account=?"); 
$edit->bind_param('ss',$username,$account); $edit->execute(); 
//报错提示：Call to a member function bind_param() on boolean in echo '<script>alert("恭喜您，注册成功！");
location.href="../login/login.html"</script>';