Makes a custom WordPress widget to display any published post.

single_post_widget.php

<?php 

// Make sure we have a theme domain for localization
	if( ! defined( 'THEME_DOMAIN' ) )
		define( 'THEME_DOMAIN', get_stylesheet() ); // Any translation has to go in your stylesheet directory

/**
 * Makes a custom widget to display any published post.
 *
 * @package WordPress
 */
class Glueckpress_Single_Post_Widget extends WP_Widget {

	/**
	 * Constructor
	 *
	 * @return void
	 **/
	function Glueckpress_Single_Post_Widget() {
		$widget_ops = array( 'classname' => 'widget_glueckpress_single_post', 'description' => __( 'Use this widget to display any single post from your published posts.', THEME_DOMAIN ) );
		$this->WP_Widget( 'widget_glueckpress_single_post', __( 'Single Post', THEME_DOMAIN ), $widget_ops );
		$this->alt_option_name = 'widget_glueckpress_single_post';

		add_action( 'save_post', array(&$this, 'flush_widget_cache' ) );
		add_action( 'deleted_post', array(&$this, 'flush_widget_cache' ) );
		add_action( 'switch_theme', array(&$this, 'flush_widget_cache' ) );
	}

	/**
	 * Outputs the HTML for this widget.
	 *
	 * @param array An array of standard parameters for widgets in this theme
	 * @param array An array of settings for this widget instance
	 * @return void Echoes it's output
	 */
	function widget( $args, $instance ) {
		global $post;
		$cache = wp_cache_get( 'widget_glueckpress_single_post', 'widget' );

		if ( !is_array( $cache ) )
			$cache = array();

		if ( ! isset( $args['widget_id'] ) )
			$args['widget_id'] = null;

		if ( isset( $cache[$args['widget_id']] ) ) {
			echo $cache[$args['widget_id']];
			return;
		}

		ob_start();
		extract( $args, EXTR_SKIP );

		// Widget settings
		$post_id            = isset( $instance['post_id'] ) ? $instance['post_id'] : 0;
		$show_post_title    = isset( $instance['show_post_title'] ) ? $instance['show_post_title'] : false;
		$show_post_content  = isset( $instance['show_post_content'] ) ? $instance['show_post_content'] : false;
		$show_post_image    = isset( $instance['show_post_image'] ) ? $instance['show_post_image'] : false;

		$post    = $content_post = get_post( $post_id );
		$content = $content_post->post_content;
		$content = apply_filters('the_content', $content);
		$content = str_replace(']]>', ']]>', $content);

		echo $before_widget;

		if ( $show_post_title ) // Post title
			echo $before_title . $content_post->post_title . $after_title . "\n"; // Post title

		if ( $show_post_image ) : // Posts image ?>

			<figure id="showcase-slider-image-<?php echo $post_id; ?>" class="showcase-slider-image">
				<?php echo get_the_post_thumbnail( $post_id, array( FEATURED_IMAGE_WIDTH, FEATURED_IMAGE_HEIGHT ) ) . "\n"; ?>
			</figure><?php

		endif;

		if ( $show_post_content )
			echo $content . "\n"; // Post content

		echo $after_widget;

		$cache[$args['widget_id']] = ob_get_flush();
		wp_cache_set( 'widget_glueckpress_single_post', $cache, 'widget' );

	}

	/**
	 * Deals with the settings when they are saved by the admin. Here is
	 * where any validation should be dealt with.
	 */
	function update( $new_instance, $old_instance ) {

		$instance = $old_instance;

		$title                         = strip_tags( $new_instance['title'] );
		$instance['post_id']           = (int) $new_instance['post_id'];
		$instance['show_post_title']   = (bool) $new_instance['show_post_title'];
		$instance['show_post_content'] = (bool) $new_instance['show_post_content'];
		$instance['show_post_image']   = (bool) $new_instance['show_post_image'];
		
		$this->flush_widget_cache();

		$alloptions = wp_cache_get( 'alloptions', 'options' );
		if ( isset( $alloptions['widget_glueckpress_single_post'] ) )
			delete_option( 'widget_glueckpress_single_post' );

		return $instance;
	}

	function flush_widget_cache() {
		wp_cache_delete( 'widget_glueckpress_single_post', 'widget' );
	}

	/**
	 * Displays the form for this widget on the Widgets page of the WP Admin area.
	 */
	function form( $instance ) {

		$title = empty( $instance['post_id'] ) ? __( 'Single Post', THEME_DOMAIN ) : get_the_title( $instance['post_id'] );
		$post_id           = isset( $instance['post_id'] ) ? $instance['post_id'] : 0;
		$show_post_title   = isset( $instance['show_post_title'] ) ? $instance['show_post_title'] : true;
		$show_post_content = isset( $instance['show_post_content'] ) ? $instance['show_post_content'] : true;
		$show_post_image   = isset( $instance['show_post_image'] ) ? $instance['show_post_image'] : true; 
		?>

		<p>
			<label for="<?php echo $this->get_field_id('post_id'); ?>"><?php echo __( 'Display Post:', THEME_DOMAIN ) ?></label>
				<select class="widefat" id="<?php echo $this->get_field_id('post_id'); ?>" name="<?php echo $this->get_field_name('post_id'); ?>">
				<?php
					$wp_query = new WP_Query( array(
						'post_type'   => 'post',
						'showposts'   => -1,
						'post_status' => 'publish'
					));
					if ( $wp_query->have_posts() ) { // begin have_posts() statement
						while ( $wp_query->have_posts() ) {
							$wp_query->the_post();
							$current_id = get_the_ID();
							$extra = $current_id == $post_id ? 'selected' : '';
							
							echo '<option value="' . $current_id . '" ' .$extra . '>' . get_the_title() . '</option>';
						}
					} else { // continuing have_posts() statement

						echo '<option value="empty">' . __( 'No posts available.', THEME_DOMAIN ) . '</option>';

					} // end have_posts() statement
					?>
				</select>
		</p>

		<input type="hidden" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" value="<?php echo $title; ?>" />
		<p>
			<input class="checkbox" type="checkbox" <?php checked( (bool) $show_post_title, true ); ?> id="<?php echo $this->get_field_id( 'show_post_title' ); ?>" name="<?php echo $this->get_field_name( 'show_post_title' ); ?>" />
			<label for="<?php echo $this->get_field_id( 'show_post_title' ); ?>"><?php echo __( 'Display title', THEME_DOMAIN ) ?></label>
		</p>

		<p>
			<input class="checkbox" type="checkbox" <?php checked( (bool) $show_post_content, true ); ?> id="<?php echo $this->get_field_id( 'show_post_content' ); ?>" name="<?php echo $this->get_field_name( 'show_post_content' ); ?>" />
			<label for="<?php echo $this->get_field_id( 'show_post_content' ); ?>"><?php echo __( 'Display content', THEME_DOMAIN ) ?></label>
		</p>

		<p>
			<input class="checkbox" type="checkbox" <?php checked( (bool) $show_post_image, true ); ?> id="<?php echo $this->get_field_id( 'show_post_image' ); ?>" name="<?php echo $this->get_field_name( 'show_post_image' ); ?>" />
			<label for="<?php echo $this->get_field_id( 'show_post_image' ); ?>"><?php echo __( 'Display featured image', THEME_DOMAIN ) ?></label>
		</p><?php

		wp_reset_query();
	}
} // end class