This is my code for login. when i try to login at 1st time it pass blank value to session.but when i go back to login page again & signin that time it successfully signin. i didnt understand whats wrong with this code?? plz help me.

session_unset();
    session_start();
    Global $i;
    $root = $_SERVER['DOCUMENT_ROOT']; 
    include($root."/signinConfig.php");
    function redirect($redirect=null)
    {
    header("Location : $redirect");
    }
           if (isset($_POST['submit']))
           {
            $check = mysql_query("SELECT * FROM user_reg WHERE `username` = '".$_POST['username']."'")or die("Dont Dare To Hack");  
            $check2 = mysql_numrows($check);
                if ($check2 == 0) 
                {   
    die('That user does not exist in our database. <a href="../Signup/registration.php" style="text-decoration: none;">Click Here To Register</a>');            
                } 
                else
                {
                    $i=0;
                    $password = mysql_result($check,$i,"password");
                    $_POST['pass'] = stripslashes($_POST['pass']); 
                    $pass = md5($_POST['pass']);  
            if ($pass == $password) 
        { 
            $_SESSION['username']= mysql_result($check,$i,"username");
            mysql_close();
            redirect("/User_CP/user_cp.php");
            } 
        else
        {
        die('Incorrect password, please try again.'); 
        }   
                }
            }//after this html code take place

config dosyasında i veritabanı bağlantı kodu saklamak & oturum kodu sona erer. işte burada

if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1000)) { // last request was more than 30 minates ago 
    session_destroy();   // destroy session data in storage 
    session_unset();     // unset $_SESSION variable for the runtime 
    } 
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp