I believe these are two very different technologies that are great in their own fields. Nodejs can scale well due to its asynchronous nature and ever growing set of libraries and framework, while php is synchronous in nature. If I'm building something that handles a large amount of traffic, I'd go with nodejs (although i'd choose a low level language such as go or rust if I'm given the freedom) as it's easier to scale. But I've seen big players (Facebook) use php in some part of their infrastructure.