onBeginContact : Phaser.Signal
This Signal is dispatched when a first contact is created between two bodies. This happens before the step has been done. It sends 5 arguments: bodyA, bodyB, shapeA, shapeB and contactEquations. It is possible that in certain situations the bodyA or bodyB values are null. You should check for thisin your own code to avoid processing potentially null physics bodies.
Source code: physics/p2/World.js (Line 182)