PHP session in logging in - php

Question

the below given is my code for a simple login page using session...
since i'm new to php or any such languages i expect any one to correct my code yours faithfully Arunkumar
if(isset($_REQUEST['btnLogin']))
{
$Uname=$_REQUEST['txtUname'];
$pass=$_REQUEST['password'];
$obj-> check_login($Uname,$pass);
$result=$obj-> executec1();
//echo'Query'.$query;
//$result=mysql_query($query);
//$row = mysql_num_rows($result);
//if($row==0)
if($result)
{
// echo "username or password is incorrect";
//}
//else
if(isset($_REQUEST['']))
{
setcookie("username", $Uname);
setcookie("password", $password);
}
else {
setcookie("username", "");
setcookie("password", "");
}
$_SESSION['username'] = $Uname;

Answer 1

session_start();
if(isset($_POST['login_id']))
{
$login='';
$login_id=$_POST['login_id'];//username coming from login form page
$password =$_POST['password'];//password coming from login form page
$query = "SELECT * FROM users WHERE login_id='$login_id' AND password='$password'";
$result = mysql_query($query);
if(mysql_num_rows($result)> 0)//if user user_id and matches then it must show 1 coderun
{
$user_id = mysql_result($result, 0, "user_id");
$screen_name = mysql_result($result, 0, "username");
$_SESSION["session_user_id"] = $user_id;/*like this you can make
different session */
$_SESSION["session_screen_name"] = $screen_name;
header("Location: home.php");
exit();
}
else
{
$errormess = "Invalid Login ID or Password";
}
}

Answer 2

You should first start session with this
session_start();
or better
if(!isset($_SESSION))
{
session_start();
}
if(!isset($_SESSION['username']))
{
$_SESSION['username']=$Uname;
}