[pmwiki-users] Losing the www. in site URL

[marc]

I have a site where I'm losing the initial www. on the URLs displayed in 
the browser's URL input field. So, www.example.com/pmwiki/Main displays 
as example.com/pmwiki/Main.

Thus I click a link to
  http://www.example.com/pmwiki
and get
  http://example.com/pmwiki/

This truncation is the same across the PmWiki part of the site.

I tend to use:

  $ScriptUrl = 'http://'. $_SERVER['HTTP_HOST'] .'/pmwiki';
  $PubDirUrl = 'http://'. $_SERVER['HTTP_HOST'] .'/pmwiki/pub';

in my local/config.php, so that I can use the same config/php locally 
for testing as on the live site. Checking the site I can see that:
  $_SERVER['HTTP_HOST']:www.example.com

Nevertheless, I changed the above to:

  $ScriptUrl = 'http://www.example.com/pmwiki';
  $PubDirUrl = 'http://www.example.com/pmwiki/pub';

What happened is that:

a link to
  http://www.example.com/pmwiki
displays
  http://example.com/pmwiki/

but the links are of the expected form:

  http://www.example.com/pmwiki/Main/WikiSandbox

Does anyone know why the www. should be 'lost' initially?

And why changing from $_SERVER['HTTP_HOST'] causes a change of 
behaviour?

Thanks.

-- 
Best,
Marc