首先在sql中字节。

ALTERTABLE`content`ADD`reply`VARCHAR(200)CHARACTERSETutf8COLLATEutf8_general_ciNOTNULLAFTER`content`

一. 新建login.php

<html xmlns="http://www.w3.org/1999/xhtml">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<body><table width="678" align="center">
<tr>
<td colspan="2"><h1>留言本</h1></td>
</tr>
<tr>
<td width="586"><a href="index.php">首页</a> | <a href="liuyan.php">留言</a></td>
</tr>
</table>
<table align="center" width="678">
<tr>
<td align="center">
<form name="form1" method="post" action="login2.php">
<label for="textfield"></label>
<p> </p>
<p>帐号：
<input type="text" name="name" id="name">
</p>
<p>密码：
<input type="password" name="pw" id="pw">
</p>
<p>
<input type="submit" name="button" id="button" value="提交">
　　
<input type="reset" name="button2" id="button2" value="重置">
</p>
</form></td>
</tr>
</table>
</body>
</html>

二.login2.php

<?
session_start();
header("content-Type: text/html; charset=utf-8");
$name = $_POST['name'];
$pw = $_POST['pw'];
if($name == "admin" && $pw == "admin"){
$_SESSION["adminname"] = $name;
echo "<script>alert('登录完成，返回首页!');location.href='index.php';</script>";
}else{
echo "<script>alert('错误!');location.href='login.php';</script>";
}
?>

三. 在原有的index.php上添加

<?php
session_start();
include("config.php");
$sql = "select * from content";
$resule = mysql_query($sql,$q);
?>
<html>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<body>
<table width="678" align="center">
<tr>
<td colspan="2"><h1>留言本</h1></td>
</tr>
<tr>
<td width="586"><a href="index.php">首页</a> | <a href="liuyan.php">留言</a></td>
<td width="80">
// 新增管理员登录
<?php
if (isset($_SESSION['adminname']) && $_SESSION["adminname"] == "admin"){
echo "<a href='logout.php'>登出</a>";
}else{
echo "<a href='login.php'>管理员登录</a>";
}
?>
</td>
</tr>
</table>
<p>
<?
while($row=mysql_fetch_array($resule))
{
?>
</p>
<table width="678" border="1" align="center" cellpadding="1" cellspacing="1">
<tr>
<td width="178">Name:<? echo $row[1] ?></td>
<td width="223">Email:<? echo $row[2] ?></td>
<td width="100">
<?php
if(isset($_SESSION['adminname']) && $_SESSION["adminname"] == "admin"){
echo "<a href='huifu.php?id=" . $row[0] . "'>回复</a>";
echo " | " . "<a href='delete.php?id=" . $row[0] . "'>删除</a>";
} else {
echo "";
}
?>
</td>
</tr>
<tr>
<td colspan="4"><? echo $row[3] ?></td>
</tr>
<tr>
<td colspan="4"><?
if($row[4] == ""){
?>
<? echo "暂无回复。";?>
<? }else {echo "管理员回复：". $row[4]; } ?></td>
</tr>
</table>
<?
}
?>
</body>
</html>

四. 新建huifu.php

<?php
/* session方式
session_start();
$_SESSION["id"]=$_GET["id"];
*/
include("config.php");
$sql = "select * from content where id=".$_GET["id"];
$resule = mysql_query($sql,$q);
/* cookie方式 */
SetCookie("id",$_GET["id"]);
session_start();
header("content-Type: text/html; charset=utf-8");
if(empty($_SESSION["adminname"])){
exit("<script language='javascript'>alert('您尚未登录后台，或登录已超时，请重新登录！');window.location.href='login.php';</script>");
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<body>
<table width="678" align="center">
<tr>
<td colspan="2"><h1>留言本</h1></td>
</tr>
<tr>
<td width="586"><a href="index.php">首页</a> | <a href="liuyan.php">留言</a></td>
</tr>
</table>
<table align="center" width="678">
<tr>
<td>
<form name="reply" method="post" action="reply.php" >
<p>回复：
<?
while($row=mysql_fetch_array($resule))
{
echo $row[3];
}
?>
</p>
<p>
<textarea name="reply" id="reply" cols="45" rows="5"></textarea>
 
</p>
<p>
<input type="submit" name="button" id="button" value="回复" />
<input type="reset" name="button2" id="button2" value="重置" />
</p>
</form>
</td>
</tr>
</table>
</body>
</html>

五. reply.php 回复留言提交页面.

<?
include("config.php");
/*
session_start();
$id = $_SESSION["id"];
/* cookie方式 */
$id = $_COOKIE["id"];
$sql = "select * from content";
header("content-Type: text/html; charset=utf-8");
$patch = $_POST["reply"];
$reply = str_replace("
","<br />",$patch);
$resule = mysql_query("UPDATE `2`.`content` SET `reply` = '$reply' WHERE `content`.`id` ="."$id");
echo "<script>alert('回复成功!');location.href='index.php';</script>";
?>

这样就可以建立出一个简单的管理.管理帐号都是admin 因为只是判别输入的是不是admin 是的话就把值输入进session中.