Cal and Todd talk about what the technology stack looks like at Answers.com, the "second largest PHP-based site" on the web. Technologies mentioned include Java (for search indexing), Varnish for caching and various database technologies. They also cover the talk he presented at this year's php[tek] (2016) about deploying large scale PHP applications.
You can listen (and watch) this episode either through the in-page audio or video player. If you'd like to listen offline, you can download the mp3 directly. If you enjoy the show, be sure to subscribe to their feed and follow them on Twitter for updates when the latest shows are released.