<?php

$username = "username";
$password = "password";

if(!empty($_GET['username']) && !empty($_GET['password']))
{
  if($_GET['username'] == $username && $_GET['password'] == $password)
  {
    // ulogovan
  }
  else
    exit();
}
else
{
?>
<div align="center">
  <form method="GET" action="<?= $_SERVER['PHP_SELF']; ?>">
    Username: <input type="text" name="username" /><br />
    Password: <input type="password" name="password" /><br />
    <input type="submit" />
  </form>
</div>
<?
}
?>