Node.js is just a JS engine executable, a bunch of APIs and a package manager. It can be used server side but I use it mostly on command line for grunt, less, typescript, coffeescript, phantom etc - basically using node and npm as a convenient way to install and manage build dependencies. Some of these tools aren't written in JS but they are wrapped by modules. Of course someone could compile most C++/C code into JS these days via Emscripten.
And yes it can be used to serve up content from a server although I wonder how much overlap it actually has with PHP. PHP is the P in LAMP and would suit data driven requests - where a server does database queries to produce the output. It's the database that determines how scalable the server is and whoever sets up the server would set their max clients accordingly.
Node.js is event driven and could potentially server hundreds or thousands of concurrent requests assuming they had little overhead in and of themselves. But if you bound it to a backend database then chances are performance is no better than LAMP with its max clients.
So I doubt the two would find themselves competing as solutions to the same problems. Either could serve out a basic website but beyond that it really depends what the site is doing which dictates the solution. Not that PHP or Node.js are the only solutions to problems by any stretch.