session_start();
if(isset($_POST['username']) || isset($_POST['password'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $_SESSION['username'] = $username;
    $_SESSION['password'] = $password;
}

if(isset($_SESSION['username']) || isset($_SESSION['password'])){
    $navbar = "1";
    $logindisplay = "0";
    $username = $_SESSION['username'];
    $password = $_SESSION['password'];
} else {
    header('Location:http://website.com/fail.php');
}

$authed = auth($username, $password);
if( $authed == "0" ){
    header('Location:http://website.com/fail.php');
}

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" );
 }

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

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

<?php

session_start();

if (isset($_POST['username']) && isset($_POST['password'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $authed = auth($username, $password);

    if (! $authed) {
        header('Location: http://website.com/fail.php');
    } else {
        $_SESSION['username'] = $username;
    }
}

if (isset($_SESSION['username'])) {
    $navbar = 1;
    $logindisplay = 0;
} else {
    header ('Location: http://website.com/fail.php');
}

if (isset($_POST['username'] && isset($_POST['password'])) {
    if (auth($_POST['username'], $_POST['password'])) {
        $_SESSION['user'] = /* userid or name or token or something */;
        header(/* to next page */);
    } else {
        // display "User credentials incorrect", stay on login form
    }
} else {
    // optionally: display "please fill out all fields"
}

if ($bAuthed)
{
     header("location: login.php");
}

// error case
header("location: error-login.php");

session_start();
if(isset($_POST['username']) || isset($_POST['password'])){
$username = $_POST['username'];
$password = $_POST['password'];
$_SESSION['username'] = $username;
$_SESSION['password'] = $password;
}

if(isset($_SESSION['username']) || isset($_SESSION['password'])){
$navbar = "1";
$logindisplay = "0";
$username = $_SESSION['username'];
$password = $_SESSION['password'];
$authed = auth($username, $password);
if( $authed == "0" ){
header('Location:http://website.com/fail.php');
}
} else {
header('Location:http://website.com/fail.php');
}

$password = $_POST['password'];

$_SESSION['username'] = $username;

$_SESSION['password'] = $password;

$logindisplay = "0";

$username = $_SESSION['username'];

$password = $_SESSION['password'];

header('Location:http://website.com/fail.php');