PHP doesn't get mentioned, because it isn't interesting. I've done a lot of PHP work, and still do. But there's not a lot of NEW going on in the PHP world. Their latest earth shattering move is namespaces. The only interesting thing about PHP's namespace support is how awful it is.

Almost all of the cool concurrency, natural language, efficiency, functional programming, and what-have-you research and development, is going on outside of the PHP community. Those are topics that HN collectively finds interesting, so PHP gets left out.

That said, just because PHP itself isn't particularly interesting, doesn't mean people don't build interesting things using it, and doesn't mean you shouldn't use it.