How to redirect from custom PHP form?

[KuJoe]

So I edited the clientareaproductdetails.tpl to add some HTML forms that post data to PHP pages. I am wondering what would be the best way to redirect back to clientarea.php?action=productdetails preferably with a message (error or success) after processing the PHP page? I tried using the header code but I get "Unauthorized Access Attempt", do I need to push some specific information back to WHMCS? Thanks!