Often you have to protect a directory or website, so that not everybody has access to it. Besides, there are several possibilities as for example using .htaccess, but the best and quickest method as it seems to me is with PHP code.

You create a file with the name login.php, with following contents:

<?php

$user = "test";
$pw = "12345";

function authenticate () {
  Header("WWW-authenticate: basic realm=\"Restricted Area\"");
  Header("HTTP/1.0 401 Unauthorized");
  echo "Restricted Area!";
  exit;
}

if (!isset($_SERVER['PHP_AUTH_USER']) || $_SERVER['PHP_AUTH_USER'] != $user || $_SERVER['PHP_AUTH_PW'] != $pw) {
  authenticate();
}
?>

Then you have to include the login.php to the top of index.php:

<?php
  include ("login.php");
?>

Thats all! It is important that there are no other index.* files in the directory.