<script>
$(function() {
    $( "#dialog-modal" ).dialog({
        height: 259,
        width: 400,
        resizable: false,
        modal: true,
        autoOpen: false
    });
    $( "#dialog-modal" ).dialog("open");
});
</script>

<div id="dialog-modal" title="Basic modal dialog">
    <form id="login">
    <input type="hidden" id="button_action" name="button_action">
    <p>Username: <input type="text" name="username" value=""></p>
    <p>Password: <input type="password" name="password" value=""></p>
    <p><input type="submit" name="existing" value="Login">
    <input type="submit" name="new" value="Register"></p>
    </form>
    <script>
$('input[type=submit]').click(function(){
    $('#button_action').val($(this).attr('name'));
  });

$('#login').submit(function(event) {
    event.stopImmediatePropagation();
    event.preventDefault();
    $.post('login.php', {data:$(this).serialize()}, function(data) {
        $( "#dialog-modal" ).dialog("close");
    });
});
</script>
</div>