Here is the way:

use session handling and ensure you include (or require) the class 
definition prior to registering the object.  That is all.

i.e.
require_once(CLASS_PATH."member.class");

if(!session_is_registered('visitor')){
  session_register('visitor');
}else{
  if(get_class($visitor) != 'member'){
    echo "This is a restricted site!";
  }
}