Probably and easy HEADER question

I confess I have not researched this very well...

How do I specify the full path to a file I want to point to with a header call? I tried using the full http://blah blah blah uri but this doesn't work well with development testing because I cannot always be connected to the www. There must be another way to do this, right?

Thanks,
M.

Maybe something like
header("Location :".$_SERVER['HTTP_ROOT']."/blah");

or it's SERVER_ROOT instead of HTTP_ROOT, can't remember now

isnt it $_SERVER['DOCUMENT_ROOT']?

probably lol I can't remember anymore

yeh, it is, just checked.