realpath returns empty string

Source: http://php.net/manual/en/function.realpath.php

Note: The running script must have executable permissions on all directories in the hierarchy, otherwise realpath() will return FALSE.

So your web server should have access also to pre-webroot directories.

I am currently experiencing this result when running PHP from command line, but not within an HTTP request. Got to be config or $_SERVER settings somewhere, will update if I find the fix.