How do I get the referrer URL?

I tried to create a plugin to get the referrer URL, but it always return nothing (rewrite rules probably caused it). I basically used this:

add_filter('init','recordhit');
function recordhit($not_used='') {
    $referer = "Referer: ".(isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : 'Unknown' );
}

And tried to echo out $referer, which didn’t work. When I used:

add_filter('init','recordhit');
function recordhit($not_used='') {
    $referer = "Referer: ".(isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : 'Unknown' );
echo $referer;
}

it still failed.

Oh and I’m aware that there’s a plugin for something like this, but I don’t want to use it as I have plans to do something else.