PHP Login with no database

<?php session_start(); // declare variables // $username = 'username'; $password = 'password'; $random1 = 'secret_key1'; $random2 = 'secret_key2'; $hash = md5($random1.$pass.$random2); $self = $_SERVER['REQUEST_URI']; // user logout // if(isset($_GET['logout'])) { unset($_SESSION['login']); } // user is logged in // if (isset($_SESSION['login']) && $_SESSION['login'] == $hash) { ?> <p>Hello <?php echo $username; ?>, you have successfully logged in!</p> <a href="?logout=true">Logout?</a> <?php } // form has been submitted // else if (isset($_POST['submit'])) { if ($_POST['username'] == $username && $_POST['password'] == $password){ // username and password are correct $_SESSION["login"] = $hash; header("Location: $_SERVER[PHP_SELF]"); } else { // display form with errors display_login_form(); echo '<p>Username or password is invalid</p>'; } } // show the login form // else { display_login_form(); } function display_login_form(){ ?> <form action="<?php echo $self; ?>" method='post'> <label for="username">username</label> <input type="text" name="username" id="username"> <label for="password">password</label> <input type="password" name="password" id="password"> <input type="submit" name="submit" value="submit"> </form> <?php } ?>