System plugin to modify form inputs.

formmod.php

<?php
/**
 * @copyright	Copyright (C) 2005 - 2013 Open Source Matters, Inc. All rights reserved.
 * @license		GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('JPATH_BASE') or die;

class plgSystemFormmod extends JPlugin
{
	/**
	 * @param	JForm	$form	The form to be altered.
	 * @param	array	$data	The associated data for the form.
	 *
	 * @return	boolean
	 * @since	1.6
	 */
	public function onContentPrepareForm($form, $data)
	{
		if (!($form instanceof JForm))
		{
			$this->_subject->setError('JERROR_NOT_A_FORM');
			return false;
		}

		// Check we are manipulating a valid form.
		if ($form->getName() !== 'com_mycomponent.site')
		{
			return true;
		}

        // Add the show_attractions field
		$type_field = new SimpleXMLElement('<field />');
		$type_field->addAttribute('type', 'radio');
		$type_field->addAttribute('name', 'show_attractions');
		$type_field->addAttribute('label', 'Allow Editing Attractions');
		$type_field->addAttribute('fieldset', 'advanced');
		$type_field->addAttribute('default', '1');
		
		$options = ['1' => 'Yes', '0' => 'No'];
		
		foreach ($options as $key => $value)
		{
			$child = $type_field->addChild('option', $value);
			$child->addAttribute('value', $key);
		}
		
		$form->setField($type_field, 'params');
		
		return true;
	}
}