function popup(url, w, h) {
    var windowHeight = h;
    var windowWidth = w;
    var centerWidth = (window.screen.width - windowWidth) / 2;
    var centerHeight = (window.screen.height - windowHeight) / 2;

    newWindow = window.open(url, 'Popup', 'resizable=0,width=' + windowWidth +
    ',height=' + windowHeight +
    ',left=' + centerWidth +
    ',top=' + centerHeight);

    newWindow.focus();
    return newWindow.name;
}