Nick,
Hope this helps. 🙂
In most browsers, form values should be remembered explicitly unless you have the default value set to an empty string. This is not really a PHP thing, it's an HTML thing. in your text input boxes, make sure that there is no value = "" field.
example:
<input type="text" name="username" value="">
will give blank results when user hits back, as the value is set to the empty string, while:
< input type="text" name="username">
should retain the user's value.
Alternatively, you can use PHP to force values to be remembered like this.
<input type="text" name="username" value="<? echo($my_value); ?">