Why does this callback get ignored:

```
new OpenLayers.Layer.Vector("My layer",
      {
        projection : "EPSG:4326",
        strategies : [ new OpenLayers.Strategy.Fixed(), refreshStrategy ],

        protocol : new OpenLayers.Protocol.HTTP(
        {            
          url    : "myphpurl.php",
          params : { rand : Math.random() * 1000 },
          format : new OpenLayers.Format.GeoJSON(),
          callback: function(){alert('test');}
        }),

```

I'm trying to get access to how many features were returned...