when does $_SERVER['HTTP_REFERER'] get set?

[sneakyimp]

i'm working on a login page. certain pages require login and redirect like this:

if ($not_logged_in) {
  header("Location: login.php");
}

in the login page, i want to do something like

if ($login_successful) {
  header("Location: " . $_SERVER['HTTP_REFERER']);
}

but i have found that redirecting using header sometimes means that http_referer is empty.

do you have to click a link for http_referer to be set? if not, i will need to specify in each and every page that requires login which page needs to be returned to.

[sid]

http://us2.php.net/manual/en/reserved.variables.php#reserved.variables.server

'HTTP_REFERER'
The address of the page (if any) which referred the user agent to the current page. This is set by the user agent. Not all user agents will set this, and some provide the ability to modify HTTP_REFERER as a feature. In short, it cannot really be trusted.

[trippleweb]

I use sessions to accomplish this type of task