Box2D has been one of the most popular physics engines in the gaming industry. Recently, Phaser has added support for this library. The Phaser team did a direct translation of Box2D's C code into JavaScript, and just added a few utilities to integrate it with Phaser API.
Take a look at the Phaser Box2D implementation page
In addition to the Box2D library, the team is working on tools for Phaser Editor that allow you to create and configure Box2D physics objects visually. It's a work in progress, but you can start creating your physics scenes now.
We highly recommend you to read the official Box2D documentation to understand the concepts and the API. The Phaser Box2D API is very similar to the original Box2D API, so you can use the official documentation to learn how to use it.