如何从parentdomain中包含一个php文件？

I am trying to include a PHP file from a parent domain in a subdomain.

In subdomain.parentdomain.com/test.php I have

include ('/var/www/vhosts/parentdomain.com/website/wp-blog-header.php');

My subdomain looks like

$_SERVER['DOCUMENT_ROOT'] gives /var/www/vhosts/parentdomain.com/subdomain.parentdomain.com

I have server access with plesk 11, but I don't know which PHP setting I have to change to enable including files from parent domain? Should I edit php_settings-> include_path setting and add /var/www/vhosts/parentdomain.com as path?

I have tried

ini_set("include_path","/var/www/vhosts/parentdomain.com/website/");
require "wp-blog-header.php";

almost working.

This one

ini_set("include_path","/var/www/vhosts/parentdomain.com/httpdocs/website/");
require "wp-blog-header.php";

seems to work when i call wordpress function bloginfo

echo bloginfo('stylesheet_url').'<br/>';
echo bloginfo('name').'<br/>';

but wordpress functions like get_the_title() doesn't work?