Its a browser issue. Browser behave differently when the back button is clicked -- this behavior mostly depends on user privacy settings. In addition, most browsers will automatically reset form upon hitting back button for pages viewed over HTTPS connection regardless of their normal behavior for HTTP connections. In addition, many HTML forms that rely heavily on AJAX do not properly -- or consistently -- restore their previous state.
Edit ----
Now that you mention using PHP and MySQL, assuming that you are also using sessions... the browser will not pre-populate the form when you hit the back button. PHP-session powered pages are not cached in browsers and hitting back button to go back to previous page is just like opening that page again from the very beginning -- most browsers will not bother restoring the form values.
