用PHP+mysql怎样搞登陆呀？？高手帮帮忙。

$sql="select user_name,user_password from tab_user where user_name='$_POST[td_username]' and user_password='$_POST[td_password]'";
$result=mysql_query($sql);
$r=mysql_fetch_array($result);
$puser_name=$r["user_name"];
$ppassword=$r["user_password"];

if($puser_name==$_POST["td_username"])
{
echo "ok!!!!!!!!!!!!!!!!!!!!!!!!!";

}
else
{
echo "密码错误,请正确输入密码";
}