This sample application for Heroku shows how ddollar/heroku-buildpack-multi can be used to combine the Node.js and PHP buildpacks, which allows using Node from inside the PHP buildpack's bin/compile.
In this example, we're using Bower in a Composer post-install-cmd to install Bootstrap.
To try this, clone this repo, heroku create --buildpack https://github.com/ddollar/heroku-buildpack-multi, then git push heroku master.
Example: http://heroku-multipack-nodejs-php-ex.herokuapp.com/