PHP登陆验证

sql写错了
if($_POST["user_name"]==""){
echo "请输入用户名";
echo "返回";
}elseif($_POST["parsword"==""]){
echo "请输入密码";
echo "返回";
}else{
$link=mysql_connect("localhost","root","")or die("不能连接数据库");
mysql_select_db("userdb",$link)or die("选择错误");
$sql="select*from user_db where username = ".$_POST['username'].'and password = '$_POST['paddword'];
$result=mysql_query($sql,$link);
$news=mysql_fetch_assoc($result);
mysql_free_result($result);
if($news['user_name']==$_POST['user_name'] && $news['parsword']==$_POST['parsword']){
echo "登陆成功";
echo "登陆";
}else{
echo "密码错误或用户名不正确";
echo "返回";
}
}
?>