Onclick event and PHP function

Hi, I have some link on which if user click, should do the following things: - open a particular page - setup to sessions some particular variable value however, PHP function should be declared before HTML headers. so i was thinking about something like that <?php function vardef($lg) { ... store lg to my session variables ... } ?> <HTML> ... <A Href="none" Onclick="<?php vardef("eng") ?>">english</A> <A Href="none" Onclick="<?php vardef("fre") ?>">french</A> ... </HTML> but this does not work because my session variable is always setup to the last value (in this case "fre"), even if i click only on english :-( could you help me with this PHP issue ? thanks a lot, Alain