web 1
<?php
if(isset($_POST['enviar'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
}
?>
perfecta
web 2
   <?php
session_start();
    if($_POST["username"] == "123" && $_POST["password"] == "123"){
        $_SESSION['username'] = $_POST['username'] ;
        $_SESSION['password'] = $_POST['password'] ;
        $_SESSION["autenticado"]= "SI";
        header ("Location: bienvenido.php");
    }
    else {
        echo "Debes loguearte";
    }
?>


web 3
perdido