My personal WordPress sidebar widget setup using PHP5 OOP

1. functions.php

<?php
	
	# Custom widgets:
	include_once('classes/THEMENAME/widgets/widgets.php');

2. widgets.php

<?php

include_once('class.foo_widget_1.php');
// ... add more custom widgets here ...
//include_once('class.foo_widget_2.php');
// ... and so on ...
// ... :) ...

3. class.foo_widget_1.php

<?php

include_once('class.foo_widget_setup.php');
include_once('class.foo_base_widget_1.php');

/**
 * This is the basic widget boilerplate.
 */

class Foo_Widget_1 extends Foo_Base_Widget_1
{
	
	public function __construct() {
		
		$this->template = array('widgets/foo-widget-1.php',);
		
		parent::__construct(
			
			'foo_widget_1',
			'Foo Widget 1',
			array(
				'classname' => __CLASS__,
				'description' => 'Foo Widget 1\'s description ...',
			)
			
		);
		
		new Foo_Widget_Setup(
			__CLASS__
		);
		
	}
	
}

new Foo_Widget_1();

4. class.foo_widget_setup.php

<?php

/**
 * @see http://www.jonloomer.com/2012/06/28/create-custom-wordpress-widget/
 * @see http://wordpress.stackexchange.com/a/80433/32387
 * @see http://wordpress.stackexchange.com/a/45920/32387
 * @see http://wordpress.stackexchange.com/a/48094/32387
 */

class Foo_Widget_Setup
{
	
	private $widget_class = '';
	private $style_admin = array();
	private $script_admin = array();
	private $style = array();
	private $script = array();
	
	private $style_defaults = array(
		'handle' => '',
		'src' => '',
		'deps' => array(),
		'version' => false,
		'media' => 'all',
	);
	
	private $script_defaults = array(
		'handle' => '',
		'src' => '',
		'deps' => array(),
		'version' => false,
		'in_footer' => false,
	);
	
	public function __construct(
		$widget_class = '',
		$style_admin = array(),
		$script_admin = array(),
		$style = array(),
		$script = array()
	) {
		
		if ($style_admin) {
			
			$this->style_admin = $style_admin;
			
			add_action('admin_enqueue_scripts', array($this, 'add_styles'));
			
		}
		
		if ($script_admin) {
			
			$this->script_admin = $script_admin;
			
			add_action('admin_enqueue_scripts', array($this, 'add_scripts'));
			
		}
		
		if ($style) {
			
			$this->style = $style;
			
			add_action('wp_enqueue_scripts', array($this, 'add_styles'));
			
		}
		
		if ($script) {
			
			$this->script = $script;
			
			add_action('wp_enqueue_scripts', array($this, 'add_styles'));
			
		}
		
		if ( ! empty($widget_class)) {
			
			$this->widget_class = $widget_class;
			
			add_action(
				'widgets_init',
				array(
					$this,
					'register_widget',
				)
			);
			
		}
		
	}
	
	public function register_widget() {
		
		register_widget($this->widget_class);
		
	}
	
	public function add_styles() {
		
		return $this->add_files('style');
		
	}
	
	public function add_scripts() {
		
		return $this->add_files('script');
		
	}
	
	private function add_files($kind = '') {
		
		$return = FALSE;
		
		if ($kind) {
			
			$files = (is_admin()) ? $this->{ $kind . '_admin' } : $this->{ $kind };
			
			if ( ! empty($files)) {
				
				foreach($files as $file) {
					
					$args = wp_parse_args($file, $this->{ $kind . '_defaults' });
					
					call_user_func_array(
						
						('wp_enqueue_' . $kind),
						array(
							$args['handle'],
							$args['src'],
							$args['deps'],
							$args['version'],
							((array_key_exists('media', $args)) ? $args['media'] : $args['in_footer']),
						)
						
					);
					
				}
				
				$return = TRUE;
				
			}
			
		}
		
		return $return;
		
	}
	
}

5. class.foo_base_widget_1.php

<?php

/**
 * Generic sidebar widget that contains a title and text field.
 *
 * See "Text widget class" in `/wp-includes/default-widgets.php`.
 *
 * @todo Option to make this a single-instance widget.
 * @todo Need constant that allows user to pick template base location.
 * @todo Make more like WPAlchemy.
 *
 * @see http://wordpress.stackexchange.com/questions/32103
 * @see http://wordpress.stackexchange.com/a/1834/32387
 * @see http://wordpress.stackexchange.com/questions/104077
 * @see https://github.com/farinspace/wpalchemy
 */

abstract class Foo_Base_Widget_1 extends WP_Widget
{
	
	protected $template = array();
	
	public function update($new_instance, $old_instance) {
		
		$instance = $old_instance;
		
		$instance['title'] = strip_tags($new_instance['title']);
		$instance['text'] = $new_instance['text']; // Unfiltered HTML.
		
		return $instance;
		
	}
	
	public function form($instance) {
		
		$instance = wp_parse_args((array) $instance, array('title' => '', 'text' => '',));
		
		$title = strip_tags($instance['title']);
		$text = esc_textarea($instance['text']);
		
		?>
		
		<p>
			<label for="<?=$this->get_field_id('title')?>">Title:</label>
			<input type="text" name="<?=$this->get_field_name('title')?>" id="<?=$this->get_field_id('title')?>" class="widefat" value="<?=esc_attr($title)?>">
		</p>
		
		<textarea rows="16" cols="20" name="<?=$this->get_field_name('text')?>" id="<?=$this->get_field_id('text')?>" class="widefat"><?=$text?></textarea>
		
		<?php
		
	}
	
	/**
	 * @see http://wordpress.stackexchange.com/a/4471/32387
	 */
	
	public function widget($args, $instance) {
		
		extract($args);
		
		$title = apply_filters(
			'widget_title',                                        // The name (`$tag`) of the filter hook.
			(empty($instance['title']) ? '' : $instance['title']), // The value which the filters hooked to `$tag` may modify.
			$instance,                                             // Additional variable passed to the filter functions.
			$this->id_base                                         // IBID.
		);
		
		$text = apply_filters(
			'widget_text',
			(empty($instance['text']) ? '' : $instance['text']),
			$instance
		);
		
		if ($text != '') {
			
			echo $text;
			
		} else {
			
			# Remember: Arguments will be available to included template.
			if ( ! empty($this->template)) include(locate_template($this->template));
			
		}
		
	}
	
}

6. foo-widget-1.php

Within each widget's template file, I have access to the <?=$title?> variable.

If someone has filled-in the "text" field, then that will trump the template and the value of $text will be echoed.

Here's an example template where I use the $title to get Twitter username; if that does not exist, I get an option from the theme's setting page.

<?php
	
	$twitter = '';
	
	if ($title) {
		
		/**
		 * Use the "title" of the widget as the Twitter username.
		 */
		
		$twitter = $title;
		
	} else {
		
		/**
		 * Get the Twitter username from the theme options.
		 */
		
		$options = get_option('THEMENAME_options');
		
		if (is_array($options) && $options['username']) {
			
			$user = get_user_by('login', $options['username']);
			
			if ($user) $twitter = get_the_author_meta('twitter', $user->ID);
			
		}
		
	}
	
?>

<?php if ($twitter): ?>
	
	<a 
		class="twitter-timeline" 
		href="https://twitter.com/<?=$twitter?>" 
		data-widget-id="347149624567558145" 
		data-screen-name="<?=$twitter?>" 
		width="100%"
	>
		Tweets by @<?=$twitter?>
	</a>
	
<?php endif; ?>

Related: https://gist.github.com/mhulse/5861847