how to pass HTTP_SESSION_VARS to functions?

I'm having a "little" trouble understanding how to pass session variables to functions. I can get the session variables to pass between pages by accessing them with HTTP_SESSION_VARS (which also protects them from being replaced by new values in URL), but not between functions. I can get the session variables to pass between functions if I don't use the HTTP_SESSION_VARS method, but then the variables are susceptible to being overwritten from the URL. How would I do this? What I have is like what's below - but I get a PHP error or no value, no matter what combination I seem to try. test.php contains the following: <? session_start(); require ("common.inc"); session_register("variable"); $HTTP_SESSION_VARS["variable"] = "test"; if (check_variable ($HTTP_SESSION_VARS["variable"])) echo "yes, $variable == test"; ?> common.inc contains the following: <? function check_session ($HTTP_SESSION_VARS["variable"]) { if ($variable=="test") return (TRUE); } ?> Thanks, Andrew Elliston