login_do.php

<?php
include "config.php";

$username = trim($_POST['username']);
$pass = md5(trim($_POST['password']));

if($username == '') { //判断用户名是否为空
	echo "<script type='text/javascript'>alert('用户名不能为空！');history.go(-1);</script>";
}

if($pass == '') { //判断密码是否为空
	echo "<script type='text/javascript'>alert('密码不能为空！');history.go(-1);</script>";
}

//访问数据库
$query = $db->select(PRE.'user',"where username='$username'");
if($db->num_rows($query) == 0 ){
    echo "<script type='text/javascript'>alert('该用户不存在！');history.go(-1);</script>";	
} else {
	while($row = $db->fetch_array($query)){
		if($row['password'] != $pass){
			echo "<script type='text/javascript'>alert('密码输入错误！');history.go(-1);</script>";
		} else {
			$_SESSION['name'] = $username;
			header("location:public/skip/skip_login_good.php");	
		}
	}
}