I am a newbie for Phaser and am following the Phaser documentation "collect stars". There is one statement: game.physics.arcade.overlap(obj1, obj2, overlapCallback, processCallback, callbackContext); I wonder what "callbackContext" means and how to provide a proper value for it? The tutorial uses "this" (again, what does this mean?), however, giving it "null" is also OK. Can anybody shed some light on it? Thanks