if ($password == $pass){
                $_SESSION["login"] = $_POST["username"];
                echo "<script type='text/javascript'> document.location = 'welcome.php'; </script>";

<?php
session_start();
if (isset($_SESSION["login"])){
    $username = $_SESSION["login"];
}else{
     echo "<script type='text/javascript'> document.location = 'login.php'; </script>";
     die;
}