Youre going to need to explain your problem a bit better and maybe post some code examples for us to really figure out what you're asking.
My impression is that you will want your form action to direct to process.php (which I believe you are doing). If validation is passed, you could set a value and than test the value with an if statement before using your header function.
Are you receiving an error? Its important to remember that all headers must be sent before any other output is produced - this could be your problem and may call from some restructure of your code and placement. Again, code examples would help alot!
Good luck!