Use WP theme in own (non WP) pages

Hi,
I just enabled a network for my site and main site and sub-domains work prfectly. But I have some problems with including WP theme in my own pages (PHP pages) specially when my page is not under a wordpress sub domain and sub folder.

In detail:
I have a (network) WP site like example.com (public_html folder)
And a subdomain in my network: sub1.example.com (public_html folder)
And a non WP sub-domain: sub2.example.com (another folder)

In PHP pages on my sub2 (non WP) I want to use wp theme (like I did before enabling network).

The problem is:
My sub2 sub-domain redirects to my main site.

I use this code to use wordpress (header and footer):

// Include WordPress
define('WP_USE_THEMES', true);
require('../public_html/wp-load.php');
get_header();
... ... ...
get_footer();