function additem(chk, id, val)
{
    $("input[name='categoria[]']").each(function (index, element) { element.remove(); });

    $('input[type="checkbox"]:checked').each(function(index, element) {
        var id = element.id;
        var value = element.id.split('_')[1];
        var newinput = '<input type="hidden" name="categoria[]" id="' + id + '" value="' + value + '">';
        console.log(newinput);
        $("#form_categorias").append(newinput);
    });
}