class billing
{

  function billing()
  {
    $this->form = new Form();
    $this->form->AddTextBox('phone_number', 'Phone Number:', new PhoneNumValidator());

    if (sizeof($_GET) == 0)
    {
      $this->form->output(); // show the form to the user
    }
    elseif (isset($_GET['validate']))
    {
      $field = $_GET['fieldname'];
      $value = $_GET['value'];
      if ($this->form->validate($field, $value))
      {
        echo '1';
      }
      else
      {
        echo '0';
      }
      echo $field;
      die();
    }
  }

}

Class PhoneNumValidator
{
function validate($data)
{
// return true or false
}
}