php sessions to authenticate user on login form

Question

I have the following code designed to begin a session and store username/password data, and if nothing is submitted, or no session data stored, redirect to a fail page.

Answer 1

what about using this to setup session

session_start();
if( isset($_POST['username']) && isset($_POST['password']) )
{
    if( auth($_POST['username'], $_POST['password']) )
    {
        // auth okay, setup session
        $_SESSION['user'] = $_POST['username'];
        // redirect to required page
        header( "Location: index.php" );
     } else {
        // didn't auth go back to loginform
        header( "Location: loginform.html" );
     }
 } else {
     // username and password not given so go back to login
     header( "Location: loginform.html" );
 }

and at the top of each "secure" page use this code:

session_start();
session_regenerate_id();
if(!isset($_SESSION['user']))      // if there is no valid session
{
    header("Location: loginform.html");
}

this keeps a very small amount of code at the top of each page instead of running the full auth at the top of every page. To logout of the session:

session_start();
unset($_SESSION['user']);
session_destroy();
header("Location: loginform.html");