The best thing to do, would be to learn javascript, as javascript is commonly used for these time based calculations etc.... if you are dead set on PHP aswell, then learn some AJAX
Easiest way would be a simple javascript countdown that starts based on the current time remaining that you generate with the php script. This way each time someone reloads a page, the time is set correctly and the javascript takes over displaying the countdown. You can integrate something like this: http://www.hashemian.com/tools/javascript-countdown.htm into the page.