<?php
session_start();
include("passwords.php");
if ($_POST["ac"]=="log") {
     if ($USERS[$_POST["username"]]==$_POST["password"]) {
          $_SESSION["logged"]=$_POST["username"];
     } else {
          echo 'Incorrect username/password. Please, try again.';
     };
};
if (array_key_exists($_SESSION["logged"],$USERS)) {
     echo "You're logged in.";
} else {
     echo '<form action="login.php" method="post"><input type="hidden" name="ac" value="log"> ';
     echo 'Username: <input type="text" name="username" /><br />';
     echo 'Password: <input type="password" name="password" /><br />';
     echo '<input type="submit" value="Login" />';
     echo '</form>';
};
?>