<form action="chuli.php" method="post">

　　<div>用户名：<input type="text" name="uid" /></div>

　　<div>密码：<input type="password" name="pwd" /></div>

　　<div><input type="submit" value="http://www.mamicode.com/登录" /></div>

</form>
chuli.php

<?php
$_post["uid"];
$_post["pwd"];
//造连接对象
$db = new MySQLi("localhost","root","123","mydb");
//写SQL语句
$sql = "select password from login shere uid=‘{$uid}‘";
//执行SQL语句
$resulat = $db->query($sql);
$n = $result->fetch_row();
//判断
if($uid!="" && $pwd !=‘‘)
{
if($n[0]==$psd)
{
　　header(location:main.php);
}
else
{
　　echo"用户名或密码错误";
}
｝
else
{
　　echo"用户名或密码不能为空";
}



/*写SQL语句，不安全的登录方式

//注入攻击
$sql = "select count(*) from login where useename=‘{$uid}‘ and passwoed= ‘{$pwd}‘";
//执行SQL语句
$result = $db->query($sql);
$n = $result->fetch_row();
if($n[0]>0)
{
　　header("location:main.php");
}
else
{
　　echo"用户名或密码错误"；
}*/