I find that having a 3 stage process works the best. Going from the form page->to the php script. Don't have this output anything. Have it do whatever DB calls need to be done, then do a header redirect to another page. This way, when they hit the Back button, it skips the php script and goes back to the form page. It doesn't hurt to put a double-check select query on the script also.
Since there's no real way to prevent people from hitting the back button, you're going to have to find ways to secure your site around this.
