PHP Sessions, IE Cache and Cookies

I have an application for filling in multi page forms online. Data from one form is carried forward to subsequent forms. this all works fine until a user comes back to a form using the History button on the browser.

What happens is that the application reads a cookie from the local machine. The cookie now has corrupt data in it which results in bogus data being shown to the user. Of someone else’s data being shown.

How can I fix this please?