I think the terminology is just getting lost because one man's backend is another man's application. Someone who is building user interfaces for a website might

I think the terminology is just getting lost because one man's backend is another man's application. Someone who is building user interfaces for a website might say PHP powers the backend, but someone writing that PHP application would say C (MySQL) is what really powers the backend. In recent times we have been hearing a lot about Twitter and Facebook's backends, both of which make heavy use of JVM-based languages despite still using PHP and Ruby respectively in the main web-facing application. I believe the article is drawing from that context to separate what the backend means. You can, of course, write your backend code in virtually any language, but PHP isn't particularly geared towards the task for a number of reasons.