Thank you for your thorough response. When I use the first example, I receive this error message:
Warning: require_once(/usr/local/apache/htdocs/includes/header.php) [function.require-once]: failed to open stream: No such file or directory in /home/mysitename/public_html/index.php on line 25
Fatal error: require_once() [function.require]: Failed opening required '/usr/local/apache/htdocs/includes/header.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/mysitename/public_html/index.php on line 25
So for some reason, the "require_once($_SERVER['DOCUMENT_ROOT']" call is not working properly on my new server.
Any ideas on how to modify this to get it working properly? You mentioned this is the proper method and I prefer it much more than using relative paths. Thank you for any suggestions.