老师，数据“添加失败，重新添加”是什么原因？
小王子，你在吗？
user_list.php
<?php
$mysqli=new mysqli('localhost','root','123456','admin');
if($mysqli->connect_errno){
die('CONNECT ERROR:'.$mysqli->connect_error);
}
$mysqli->set_charset('utf8');
$sql="SELECT id,username,age FROM db";
$mysqli_result=$mysqli->query($sql);
if($mysqli_result && $mysqli_result->num_rows>0){
while($row=$mysqli_result->fetch_assoc()){
$rows[]=$row;
}
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<body>
<h2>用户列表-<a href="addUser.php">添加用户</a></h2>
<table border="1" cellpadding="0" cellspacing="0" width="80%" bgcolor="#ABCDEF">
<tr>
<td>编号</td>
<td>用户名</td>
<td>年龄</td>
<td>操作</td>
</tr>
<?php $i=1;foreach($rows as $row):?>
<tr>
<td><?php echo $i;?></td>
<td><?php echo $row['username'];?></td>
<td><?php echo $row['age'];?></td>
<td><a href="editUser.php?id=<?php echo $row['id'];?>">更新</a><a href="do_Action.php?act=delUser&id=<?php echo $row['id'];?>">删除</a></td>
</tr>
<?php $i++;endforeach;?>
</table>
</body>
</html>
do_Action.php
<?php
header('content-type:text/html;charset=utf-8');
//接收页面
$mysqli=new mysqli('localhost','root','123456','admin');
if($mysqli->connect_errno){
die($mysqli->connect_error);
}
$mysqli->set_charset('utf8');
$username=$_POST['username'];
$username=$mysqli->escape_string($username);//escape_string转义特殊字符///的SQL语句中使用字符串
$password=md5($_POST['password']);
$age=$_POST['age'];
$act=$_GET['act'];
$id=$_GET['id'];
//根据不同操作完成不同功能
switch($act){
case 'addUser':
//echo '添加用户的操作';
$sql="INSERT db(username,password,age) VALUES('{$username}','{$password}','{$age}')";
$res=$mysqli->query($sql);
if($res){
$insert_id=$mysqli->insert_id;
echo "<script type='text/javascript'>
alert('添加成功，网站的第{$insert_id}位用户');
location.href='user_list.php';
</script>";
}else{
echo "<script type='text/javascript'>
alert('添加失败，重新添加');
location.href='addUser.php';
</script>";
}
break;
case 'delUser':
//echo '删除记录'.$id;
$sql="DELETE FROM db WHERE id=".$id;
$res=$mysqli->query($sql);
if($res){
$mes='删除成功';
}else{
$mes='删除失败';
}
$url='user_list.php';
echo "<script type='text/javascript'>
alert('{$mes}');
location.href='{$url}';
</script>";
break;
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<body>
</body>
</html>
addUser.php
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<body>
<h2>添加用户</h2>
<form action="do_Action.php?act=addUser" method='post'>
<table border="1" cellpadding="0" cellspacing="0" bgcolor="#ABCDEF" width="80%">
<tr>
<td>用户名</td>
<td><input type="text" name="username" id="" placeholder="请输入合法用户名" required="required"/></td>
</tr>
<tr>
<td>密码</td>
<td><input type="password" name="password" id="" placeholder="请输入密码" required="required"/></td>
</tr>
<tr>
<td>年龄</td>
<td><input type="number" name="age" id="" min='1' max='125' placeholder="请输入合法年龄" required="required"/></td>
</tr>
<tr>
<td colspan='2'><input type="submit" value="添加用户"/></td>
</tr>
</table>
</form>
</body>
</html>
你得放代码，那是你自己写的语句，看自己写的一句怎么能知道是什么原因呢