Install and configure HHVM on an Ubuntu 14.04 Server set up using an Apache Virtual Hosts configuration; run traditional and HHVM-based PHP-powered web applications side-by-side on the same system.
People add and remove "www." from URLs like they're optional, so you need to handle both cases. Whatever varition's chosen should send you to the same content as the other but this can lead to content duplication issues. With a few easy edits to your Apache-based site's .htaccess file, you can handle all user and crawler cases ideally.