session_unset() seems to undefine non-session variables. It happens with register_globals = off. It seems to behave just like if register_globals was on.

Example:

$domain = $_REQUEST["domain"];
if ($domain){
  echo $domain;  // correct
  session_start();
  session_unset();
  echo $domain;  // empty
  $_SESSION["domain"] = $domain;
};

So the problem seem to be: if a previous session variable is named "abc" (thus $_SESSION["abc"]), session_unset() will clear $_SESSION["abc"] but also the variable $abc which should not be in relation with it since register_globals is off.