finding the right path, using file_get_contents

Hallo,

I really mess arround hours now with finding the right path to use file_get_contents
The Idea was to create a workarround for a include which is not possible in WP with PHP. So I wrote following function in functions.php to add a Shortcode

function laden_offen_snippet() {
	$laden_offen = file_get_contents(dirname(__FILE__) . './offen.php');
	echo $laden_offen;
}
add_shortcode('offen', 'laden_offen_snippet');

and I get this warning
Warning: file_get_contents(/customer/xxxxx_xxx/web/wordpress/wp-content/themes/designpile./offen.php)
[function.file-get-contents]: failed to open stream: No such file or directory in /customer/xxxxx_xxx/web/wordpress/wp-content/themes/designpile/functions.php on line 7

Of course I get this wrong because it gives me back the full path, but I don’t know how to get the right one in there ??
as I wrote this in the functions.php of the theme this is located here:
/customer/xxxxx_xxx/web/wordpress/wp-content/themes/designpile/
The file I want to include is is located at /web/wordpress/offen.php, meaning in such a way I would have included it from within an html file.

I have searched arround and did experiments with abspath and other stuff and I’m still lost (cause I’m not an real coder ??

may someone can assist me there, please?
Thanks a lot
Tom