Way to have a theme-settings form input to accept numbers only?

Question

I am creating some theme settings and am creating a form to populate them in theme-settings.php. I am using the hook hook_form_system_theme_settings_alter().

I would like to restrict or verify that only numbers can be entered into a field. I've searched online but have not found how to do this. Would anyone know what I ought to add to my code?

Code currently looks like:

$form['mysite_theme']['mysite_contact_phone'] = array(
    '#type' => 'textfield',
    '#title' => t('Contact Number'),
    '#default_value' => $default['mysite_contact_phone'],
    '#description' => t('Enter the main contact number for this site.'),
    '#required' => TRUE,
  );

Answer 1

You should be able to add your own validation handler. After the code you've shown, add this:

$form['#validate'][] = 'my_validation_handler';

Then in your handler, you can add your own validation:

function my_validation_handler($form, &$form_state)
{
  if(!is_numeric($form_state['values']['mysite_contact_phone']))
  {
    form_set_error('mysite_contact_phone', t('Contact number must be numeric'));
  }
}

Answer 2

You can use Elements module to get a HTML5 number field.