How do I add an overlay to my OpenStreetMap

Question

I currently use a google map and create tiles for it by using a PHP script to generate the required PNG images

 var myTileLayer = new GTileLayerOverlay(
                                new GTileLayer(null, null, null, {
                                    //tileUrlTemplate: 'Overlays/Overlay_{Z}_{X}_{Y}.png',
                                    tileUrlTemplate: 'gentile.php?Zoom={Z}&TileX={X}&TileY={Y}',
                                    isPng:true,
                                    opacity:1.0
                                })
                            );

                                map.addOverlay(myTileLayer);

I now wish to use OpenStreetMap and according to this source: http://wiki.openstreetmap.org/wiki/OpenLayers_Simple_Example

I need to use this code to acheive that, however it seems to want me to use premade tiles when I want to gen them on the fly

 var newLayer = new OpenLayers.Layer.OSM("New Layer", "URL_TO_TILES/${z}/${x}/${y}.png", {numZoomLevels: 19});
 map.addLayer(newLayer);

Really I need to use gentile.php?Zoom={Z}&TileX={X}&TileY={Y}