php 用户登录验证

<?php
session_start();
include("DBDA.php");
$db=new DBDA();

$uid=$_POST["uid"];
$pwd=$_POST["pwd"];

$sql="select pwd from users where uid='{$uid}'";

$mima=$db->StrQuery($sql);

if($pwd==$mima)
{
$_SESSION["uid"]=$uid;
header("location:zhuye.php");
}
else
{
echo "用户名或密码错误";
}