Grab referrer information

[goldenllama]

I'm editing one of my templates that has a form in it and I'd like for it to grab the referrer information (the url of the thread from which the user came) and print it as the default value in one of the form fields. Right now I have:

$reflink = $_SERVER['HTTP_REFERER'];
<input type="text" size="30" name="field" value="$reflink" />

But it doesn't seem to work. Does vB happen to have the referrer information already stored as a variable that I can use without having to define? Am I defining it wrong? Am I calling it wrong? I feel like an imbecile for not being able to figure this out.