var style = new OpenLayers.Style({ pointRadius: "${radius}", fillColor: "#ffcc66", fillOpacity: 0.8, strokeColor: "#cc6633", strokeWidth: 1, strokeOpacity: 0.8, label: "${getLabel}" }, {
    context: {
        getLabel: function(feature) {
                if (feature.cluster) {
                    if (feature.cluster.length > 1) {
                        return feature.cluster.length;
                    }
                }
                return '';
        },
                radius: function(feature) {
                    var pix = 2;
                    if(feature.cluster) {
                        pix = Math.min(feature.attributes.count, 7) + 3;
                    }
                    return pix;
                }
    }
});

strategy = new OpenLayers.Strategy.Cluster();

featureSubVector = new OpenLayers.Layer.Vector("Clusters", {
    strategies: [strategy],
    styleMap: new OpenLayers.StyleMap({ "default": style, "select": { fillColor: "#8aeeef", strokeColor: "#32a8a9" } })
});

var selectControl = new OpenLayers.Control.SelectFeature( featureSubVector, {hover: true} );
map.addControl(selectControl);
selectControl.activate();
map.addLayer(featureSubVector);