Front page validation to PHP

We use Frontpage to create forms and to add validation to data entry. These have been saved as html and put on a hosted server running apache and PHP. They work fine. Now we have added cookies using PHP code to the forms and renamed them .php rather than html. The cookies work fine so does data capture, but the validation no longer works. Hence an enquirier can simply press submit and the form gets submitted with no data if they have chosen not to enter any information. What do I need to do to our Frontpage forms to ensure the validation works again and we can force data to be entered. Any ideas welcome. Thanks, Graham