I did what one of the other posters suggested and posted the variables back to the form through a url
For example if the user missed out their first name, but filled in the surname, the page would ask them to fill in their first name.
A link back would be provided back to the form
The url would look like this following on the page if a user looked at the source code
<a href="form.php?name=&surname=phpbuilder">Click here to go back</a>
So when the user clicks back the input field surname would show phpbuilder
The .php code would be
<a href="form.php?name=<? echo $first_name; ?>&surname=<? echo $surname; ?>">Click here to go back</a>