I want to use PHP (Symfony2) because building apps with it is quicker than building apps w node (IMO). I want the speed and scalability of node, however. My proposed solution is to use node to handle all of the app's data. I would then use Symfony2 as a client to node and pull any data I need from the node server's REST API. I could then connect to node directly with my iOS/android app and take advantage of its scalability. Is this a viable solution? How can this be improved?
If I understand you correctly you want to know.
a). If you can build a API server with nodejs. The obvious answer is yes.
b). If your PHP app can get data from the nodejs api server. Again yes you can do this.
c). If your mobile app can get data from the nodejs api server. 3rd time yes.
But understand that you now have to deal with two separate server-side technologies\langauages.
You will have to weight-up if this "quicker" or "more maintainable" then building both applications using the same stack.
What makes Symfony or any Rails inspired framework fast to develop is the scaffolding. By using Node for the backend to handle the data management (API), you are loosing all of that, and keeping only the VC (View Controller) of Symfony's MVC. And you won't get any faster if you only keep those and add the mess of having 2 backends.
They are some Rails inpired frameworks for Node.js out there. Railway is one of them.