<link href="/css/stylesheet.css?<?php echo time(); ?>" rel="stylesheet" type="text/css" />
This results into something like this.
<link href="/css/stylesheet.css?1281262375" rel="stylesheet" type="text/css" />
Please note that this is undesirable for a production environment because you want fast load times in production. So only use this during the development or testing phases for your website or web application.