WordPressでプラグインなしでPHP使用可能なウィジェットが利用可能になるサンプルコード

<?php
/**
* PHP_Code_Widget
* @access
* @param
* @return
* @author
* @see
*/

class PHP_Code_Widget extends WP_Widget {
	function __construct() {
		load_plugin_textdomain( 'php-code-widget', false, dirname( plugin_basename( __FILE__ ) ) );
		$widget_ops = array('classname' => 'widget_execphp', 'description' => __('Arbitrary text, HTML, or PHP Code', 'php-code-widget'));
		$control_ops = array('width' => 400, 'height' => 350);
		parent::__construct('execphp', __('PHP Code', 'php-code-widget'), $widget_ops, $control_ops);
	}

	function widget( $args, $instance ) {
		extract($args);
		$title = apply_filters( 'widget_title', empty($instance['title']) ? '' : $instance['title'], $instance );
		$text = apply_filters( 'widget_execphp', $instance['text'], $instance );
		echo $before_widget;
		if ( !empty( $title ) ) { echo $before_title . $title . $after_title; }
			ob_start();
			eval('?>'.$text);
			$text = ob_get_contents();
			ob_end_clean();
			?>
			<div class="execphpwidget"><?php echo $instance['filter'] ? wpautop($text) : $text; ?></div>
		<?php
		echo $after_widget;
	}

	function update( $new_instance, $old_instance ) {
		$instance = $old_instance;
		$instance['title'] = strip_tags($new_instance['title']);
		if ( current_user_can('unfiltered_html') )
			$instance['text'] =  $new_instance['text'];
		else
			$instance['text'] = stripslashes( wp_filter_post_kses( $new_instance['text'] ) );
		$instance['filter'] = isset($new_instance['filter']);
		return $instance;
	}

	function form( $instance ) {
		$instance = wp_parse_args( (array) $instance, array( 'title' => '', 'text' => '' ) );
		$title = strip_tags($instance['title']);
		$text = format_to_edit($instance['text']);
?>
		<p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:', 'php-code-widget'); ?></label>
		<input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>" /></p>

		<textarea class="widefat" rows="16" cols="20" id="<?php echo $this->get_field_id('text'); ?>" name="<?php echo $this->get_field_name('text'); ?>"><?php echo $text; ?></textarea>

		<p><input id="<?php echo $this->get_field_id('filter'); ?>" name="<?php echo $this->get_field_name('filter'); ?>" type="checkbox" <?php checked(isset($instance['filter']) ? $instance['filter'] : 0); ?> /> <label for="<?php echo $this->get_field_id('filter'); ?>"><?php _e('Automatically add paragraphs.', 'php-code-widget'); ?></label></p>
<?php
	}
}

add_action('widgets_init', 'php_code_widget_register');
function php_code_widget_register() {
	register_widget('PHP_Code_Widget');
}

?>