Login to WordPress as administrator (or any other user) without knowing the username or password.

wordpress-force-login-as-admin.php

<?php
include('wp-config.php');

$user_id	=	1; // Default admin user id. (usually the first user entered into the database)
$remember	=	1; // Remember Session (14 days approx)
$secure		=	(!isset($_SERVER['HTTPS']) || $_SERVER['HTTPS'] != 'on') ? 0 : 1; // Use SSL

// Force authorization as the above user without password
wp_set_auth_cookie( $user_id, $remember, $secure );

header('location: ./wp-admin/');