'make calls to php scripts'
You are always going to have the same bottlenecks if you use the same approach + NodeJS. In this case, your PHP scripts will still not provide a higher (event-based) concurrency just because you are using socket.io...In fact, if you are making a request to a PHP script for each request you receive with NodeJS, it can only be just as fast or slower than using plain-old-PHP.
