使用用户名，密码登录，正确可以登入，错误返回登录界面
1.建立登录界面
<body>
<h1>登录</h1>
<form action="chuli.php" method="post">
<div>用户名：<input type="text" name="uid" /></div>
<div>密码：<input type="text" name="pwd" /></div>
<div><input type="submit" value="登录" /></div>
</form>
</body>
2.chuli.php页面：
<?php
session_start();
$uid = $_POST["uid"];
$pwd = $_POST["pwd"];
include("../DBDA.class.php");
$db = new DBDA();
$sql = "select count(*) from users where uid =‘{$uid}‘ and pwd = ‘{$pwd}‘";
$r = $db->StrQuery($sql);
if($r==1)
{
$uid = $_SESSION["uid"];
header("location:main.php");
}
else
{
header("location:log.php");
}
3.main.php页面：
<body>
session_start();
//判断用户名是否为空，防止漏洞
if(empty($_SESSION["uid"]))
{
header("location:log.php");
}
echo $_SESSION["uid"];
</body>
原文：http://www.cnblogs.com/dianfu123/p/5528159.html