As much as I'd love to say "rewrite all the PHP in Node too" :-) yes, your description will work. After a few years in production it seems to me that Node's best position is right between the browser and the backend. Translating legacy APIs into modern, easy-to-use services.

In your case you might want to add some web hooks for the PHP application to let your Node layer know that things have happened which you can then send over Socket.IO. There may be a more efficient way to do this by tapping into the database directly though I don't tend to like going that route usually but that may just be preference.