[support] API invoke during page refresh

Hi List I want to know is there any API in D7 is invoked when a page loads for first time or during page refresh/reload. During anyother time that particular api should not be invoked. I have a custom form, and I keep certain variables in $_SESSION. During first time page loads I want to reset the session variables. Also during page refresh (for windows IE F5) also I want to reset session variables. Best Regards Kamal Net Cloud Systems Bangalore