var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var handlerA = new Cesium.ScreenSpaceEventHandler(scene.canvas);
  handlerA.setInputAction(function(click) {
    var pickedObject = scene.pick(click.position);
    if (Cesium.defined(pickedObject)) {
      $.ajax({

        type: 'GET',
        url: 'InfoBox.php',
        data: (pickedObject.id.id).val(),

        success: function(result) {
          pickedObject.id.description = "The temperatur of " + result.town + " is " + result.temp + " degrees...";
        }
      }); 
    }
  }, Cesium.ScreenSpaceEventType.LEFT_CLICK);

pickedObject.id is the entity.