Limit the characters of $referral taked from server

I'm including the referral into an xml query :

if(isset($_SERVER['HTTP_REFERER'])) $REFERRER = $_SERVER['HTTP_REFERER']; else $REFERRER = "";

$affilDataValue .= "&ref=".rawurlencode($REFERRER);

Sometimes the referral sent in the xml is more then 500 characters...there is a way to truncate it after some characters?

Thanks,

Andrea