submit.php

<?php
    session_start(); // Use on any page that uses $_SESSION.

    $username = $_POST["username"];
    $password = $_POST["password"];

    $_SESSION["username"] = $username;
    $_SESSION["password"] = $password; // Probably a bad idea. Don't do this.

    echo("Username: " . $username . " password: " . $password);
    echo("Username: " . $_SESSION["username"] . " password: " . $_SESSION["password"]);

    session_destroy(); // Use this to clear the session data (log out).