On the PHP in Action blog Dagfinn Reiersol shares a method to send messages across a POST request that's followed by a redirect (versus a simple GET where it can be in the URL).
When processing a GET request, you can display whatever messages you want. The most simplistic way is to echo them directly; or if just slightly more sophisticated, set it in the template that's about to become the web page. When processing a POST request that is to be followed by a redirect, you can't do that. The response (redirect) sent back to the browser does not have any text or HTML content.
He points out two different ways to handle the problem - either manually append the message to the URL you're redirecting to or (a bit better method) store it in a session variable and remove it once its done. Several frameworks call this a "flash message". He gives examples of how to set this in two popular frameworks - Zend Framework and CakePHP.