Call get_header() in phpbb

Hello everyone. I hope someone can help me.
I have:
working WP. In root is a folder ‘forum’, where is installed phpbb. Both are working nicely.

I need:
print header from WP in header of phpbb. It looks pretty simple, but…
1) get_header() doesn’t work because phpbb doesn’t know anything about WP
2) when i try include smth from WP, i have problems with headers.
ex. in phpbb file ‘header_overall’ a write some php like:
require_once(../wp-config.php);
it calls other friends like ‘wp-includes/functions.php’
when this file is included, i have this:
Cannot modify header information – headers already sent by (output started at /cache/tpl_prosilver_overall_header.html.php:1)
3) i can not avoid WP functions, because header contains navigation, which has links to every category on the site

I ask:
for help… Please…