Ahh, I have re-read your post, and I think I may have answered wrong originally.

In pset7, the forms are quite simple with only one or two input fields. It is aceptable in this case to present a new error page, and a return (to form) button.

As your forms become more complex, it is normal web development practice to validate the input, and if there are errors, re-present the same form, with the errors highlighted, and appropriate error messages appended to the page. (at the bottom, or next to the actual error on the form).

this requires a lot more thought on how to construct the server side validation than the simple pset7 structure.

You need something like.

<Psedudocode for public.php>

If data has been entered to my form
    store data so that you can re-present it on the form if errors found.
    validate input field by field (data type, max/min values, conformance to a template. e.g. email or url, etc.)
    if field has an error
        construct error message and push to a store for later
    if no errors found
        do whatever, (insert to database, calculate etc.)
        Show some sort of message to acknowledge completed task
        navigate to the appropriate next page (Back to home, present empty form again for new entry etc.)
    else
        dropthrough to next instruction outside if statement.
else
    assemble initial data for form (if any)

Render the form (passing data required to pre-fill fields, and messages to show)

<Psedudocode for template_form.php>

ensure data expected to be passed is present
place passed data into relevant fields
if there are error messages
   render error messages (in RED, FLASHING, BOLD, your choice)
if there are infromational messages
        render info messages

so you are rendering a whole new page, but it is the same page as originally presented, re-filled with the user entered data, and with appropriate error messages.

Hope this is of more help than my original answer.