Destroying Sessions... (Page Expired....)

I have my Sessions Started in a different function... I have a different function displaying a form dependent on input... At the end of the body of the Script (The Script that is not part of any function //based on an online example by: URS Gehrig... I try to destroy the session.. This isn't working i.e when I click on the Back Button on the Browser it returns to the initial page (it should read Page Expired)... Any ideas... Here is how its going... SessionRegistration...//returns 1 on successful Session Registration session_start()... ShowForm Function... if Session Successfully Registered.. Do something and then kill the Session else Show Form again...