<form method="post" action="login.php"><!-- ?mode=login -->

<p><input name="username" type="text" id="username" /></p>

<p><input name="password" type="password" id="password" /></p>

<p><input name="login" class="mainoption" value="Log in" type="submit"></p>

</form>

---

<?php
define('IN_PHPBB', true);
$phpbb_root_path = "../../../phpBB3/"; //Path to forum (change this)
$phpEx = substr(strrchr(__FILE__, '.'), 1);
include($phpbb_root_path . 'common.' . $phpEx);

// Start session management
$user->session_begin();
$auth->acl($user->data);
$user->setup();

if($user->data['is_registered'])
{
    //Already logged in
}
else
{
    $username = request_var('username', '', true);
    $password = request_var('password', '', true);
    $autologin = (!empty($_POST['autologin'])) ? true : false;

    $result = $auth->login($username, $password, $autologin);

    if ($result['status'] == LOGIN_SUCCESS)
    {
        //Right
    }
    else
    {
        //Wrong
    }
}
?>