I did a lot of searching and also read the PHP $_SERVER man page. Do I have this right regarding which to use for my PHP scripts for simple link definitions used throughout my site?

$_SERVER['SERVER_NAME'] is based on your web servers' config file (Apache2 in my case), and varies depending on a few directives: (1) VirtualHost, (2) ServerName, (3) UseCanonicalName, etc.

$_SERVER['HTTP_HOST'] is based on the request from the client.

Therefore, it would seem to me that the proper one to use in order to make my scripts as compatible as possible would be $_SERVER['HTTP_HOST']. Is this assumption correct?

Followup comments:

I guess I got a little paranoid after reading this article and noting that someone said "they wouldn't trust any of the $_SERVER vars":

http://markjaquith.wordpress.com/2009/09/21/php-server-vars-not-safe-in-forms-or-links/

and also:

http://www.php.net/manual/en/reserved.variables.server.php (comment: Vladimir Kornea 14-Mar-2009 01:06)

Apparently the discussion is mainly about $_SERVER['PHP_SELF'] and why you shouldn't use it in the form action attribute without proper escaping to prevent XSS attacks.

My conclusion about my original question above is that it is "safe" to use $_SERVER['HTTP_HOST'] for all links on a site without having to worry about XSS attacks, even when used in forms.

Please correct me if I'm wrong.