Zooming to WMS layer after CQL filter in OpenLayers?

I created a layer that contains points and I added it to a map as WMS layer in OpenLayers. I did a CQL filter and it works but I'd like to zoom to the selected point when I worked with getextent or getdataextent it doesn't zoom to the right place. It missed with a few Km. Here is my the code of filter:

function filterNom(){
            var filterType ="cql";
            var filter = document.getElementById('batima').value;
            // by default, reset all filters
            var filterParams = {cql_filter: null};
            if (OpenLayers.String.trim(filter) != "") {

                if (filterType == "cql") 
                    filterParams["cql_filter"] = "batiment='"+filter+"'";

            }
            // merge the new filter definitions
            wfs_layer.mergeNewParams(filterParams);
            var bounds =  new OpenLayers.Bounds
            bounds = wfs_layer.getExtent()
            map.setCenter(wfs_layer.getExtent(),18);
            var center = map.getCenter();

            center.transform(new OpenLayers.Projection("EPSG:4326"), new OpenLayers.Projection("EPSG:900913"));
            alert(center.toString());
}

and here is the code to add the WMS layer:

  wfs_layer = new OpenLayers.Layer.WMS("Patrimoines",
        "http:// localhost/geoserver/cite/wms", 
        {layers: 'cite:patrimoine' , transparent: true },
        {isBaseLayer: false, opacity: 1, singleTile: true, visibility: true,
         projection: 'EPSG:900913'});

Answer

Your wms layer is a image, so the zooming to it will not work. I set up a vector layer with a protocol and filter and then am able to zoom to the particular feature. I did some cutting and pasting but I think I got the important parts. Let me know if this is sufficient

filter = new OpenLayers.Filter.Comparison({
    type: OpenLayers.Filter.Comparison.EQUAL_TO,
    property: "field name",
    value: 123
});

protocol = {
    version: "1.1.0",

    srsName: "EPSG:900913",
    url: your url,
    featurePrefix: "prefix",
    featureNS: namespace,
    featureType: featuretype,
    geometryName: "geom"
};

vector_LAYER = new OpenLayers.Layer.Vector("vector", {
    strategies: [new OpenLayers.Strategy.Fixed()],

    styleMap: styleMap,
    filter: filter,
    protocol: protocol
});

vector_LAYER.events.on({
    featureadded: function() {
        map.zoomToExtent(vector_LAYER.getDataExtent());
    }
});

Then somewhere down down in my code I set the filter and refresh the layer.

filter.value = filter;
vector_LAYER.refresh();