wp-blog-header.php not found

I've written a PHP file that I use as a template for a WordPress page and  I want to refer to some WordPress functions on the page so at the top I have:
require_once("/nas/wp/www/bb2/wp-blog-header.php");
 global $wpdb;

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"/nas/wp/www/bb2/"  is the root of my site and I can clearly see that wp-blog-header.php is there in the root:
Site root
Yet, when the PHP file runs it throws the error:

              Warning: require_once(/nas/wp/www/bb2/wp-blog-header.php):  failed to open stream: No such file or directory . . .

Why doesn't it see wp-blog-header.php?

I have a staging copy of the site (the site is hosted on WP Engine, where it's easy to create staging sites) and it works there!  The only difference is that I included "staging" in the path to wp-blog-header.php:

 require_once("/nas/wp/www/staging/bb2/wp-blog-header.php");
 global $wpdb;

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Does anyone have any idea what could be going on?  After half a day looking at this, I'm out of ideas.

Thanks
stevaAsked:
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Dave BaldwinFixer of ProblemsCommented:
Where is the page that is 'including' that file?
stevaAuthor Commented:
The php file that has the "require()" is a bit large (750 lines) to include here but I can tell you it's a direct copy of the file in the staging version of the site - that works - except the staging copy has "staging" in the require path.  I copied the staging version and then deleted "staging" from the require path to get the live version, that doesn't work.
Dave BaldwinFixer of ProblemsCommented:
I didn't want to see it, I wanted to know what directory it is in.  Looks to me like it should probably be just...
require_once("wp-blog-header.php");

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David SankovskySenior SysAdminCommented:
First of Dave might have a good point.
Also, I'd check the ownership and the permissions on the file in question.
stevaAuthor Commented:
I found the problem.  phpinfo() shows that the document root for the live site is 

/nas/wp/www/sites/bb2

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not 
/nas/wp/www/bb2

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require_once("/nas/wp/www/sites/bb2/wp-blog-header.php");

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works.
Thanks for your input.  I split the points.
Dave BaldwinFixer of ProblemsCommented:
You're welcome, glad to help.
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