php中注册信息保存到数据库后,怎样用一个登录页面判断登录的信息与数据库的是

<?php
if(isset($_POST["submit"]) && $_POST["submit"] == "登陆")
{
$user = $_POST["username"];
$psw = $_POST["password"];
if($user == "" || $psw == "")
{
echo "<script>alert('请输入用户名或密码！'); history.go(-1);</script>";
}
else
{
mysql_connect("localhost","root","");
mysql_select_db("vt");
mysql_query("set names 'gbk'");
$sql = "select username,password from user where username = '$_POST[username]' and password = '$_POST[password]'";
$result = mysql_query($sql);
$num = mysql_num_rows($result);
if($num)
{
$row = mysql_fetch_array($result); //将数据以索引方式储存在数组中
header("location:QA.php");
echo $row[0];

}
else
{
echo "<script>alert('用户名或密码不正确！');history.go(-1);</script>";
}
}
}
else
{
echo "<script>alert('提交未成功！'); history.go(-1);</script>";
}

?>
这里提供个参考本回答被网友采纳