authenticate.php?username={$_GET['username']}&password={$_GET['password']}


if ($_GET['username'] == "secret_username" && password == "secret_password")
{
   $_SESSION['user'] = $username;
   header("Location: password_protected_page.php");
   exit;
}

<?php

    $filename = "/home/users.txt";
    $file = fopen( $filename, "r" );
    $display = fread( $file, filesize( $filename ) );
    fclose($file);

?>

/root/users.Joe.php
/root/users/Juan.php
/root/users/Tim.php

<?php
$userpath='/var/www/html/users/Tim';
$password='Timspassword';
?>

<?php
include '/root/users/'.$_POST[username];
if ($password == $_POST['password'])
{
$_SESSION['loggedin']='yes';
$_SESSION['expire']='<how much time you need?>';
}
?>