Unfortunately this cannot be done with PHP alone. You will need to use a combination of Javascript and PHP to achieve this using Ajax (google Ajax PHP for more info). Essentially, your backend PHP script sends the updated text/html/etc using Ajax or more specificly, XML-RPC to the Javascript which then dynamicly updates the webpage without the need for a reload/refresh.

A few little tutorials / guides to get you started:

'[php] Rasmus' 30 second AJAX Tutorial - [was Re: [php] AJAX & PHP]' - MARC
PHPBuilder.com, the best resource for PHP tutorials, templates, PHP manuals, content management systems, scripts, classes and more. (good diagram to explai the process)

Alan.