"PHP is interpreting the path as "/your/define/php_includes/directory//commonheader.inc". "
not really, it is interpreting the file as "/commonheader.inc", nothing more, because this include statement contains an absolute path. The 'include_path' is only used when there is no mention of a path in the include() statement.
The real point is:
The 'root' directory of your webserver is the documentroot where you keep your html pages and PHP scripts. ('/www/bladebla/webserver/documentroot/html/')
but:
the 'root' directory for PHP is the root directory of your filesystem. ('/')
Remember: include is a FILESYSTEM operation!