Redirect IE8 users to another page
I know you specified PHP, but here's a HTML/JS solution:
<!--[if IE]> <script type="text/javascript"> window.location = "http://www.google.com/"; </script><![endif]-->Taken from this answer.
If you specifically only want to redirect IE 8 users, change <!--[if IE]> to <!--[if IE 8]>.
Yes, in PHP you can use $_SERVER['HTTP_USER_AGENT'] to detect the browser and redirect accordingly.In javascript you can you navigator.userAgent to apply the same thing.