Example WordPress 5 widget to display a configurable number of event entries from the database, originally populated through a new custom post type

listing-20-1.php

<?php
/**
 * @package WH_Event_Widget
 * @version 1.0.0
 */
/*
Plugin Name: WH Event Widget
Plugin URI: https://wordpress-handbuch.com
Description: Anzeige aktueller Veranstaltungen
Author: Johannes Mustermann
Version: 1.0.0
Author URI: https://ihredomain
*/

class WH_Event_Widget extends WP_Widget {
	public function __construct() {
		$widget_options = array(
			'classname'   => 'wh_event_widget',
			'description' => 'Anzeige aktueller Veranstaltungen',
		);
		parent::__construct( 'wh_event_widget', 'Aktuelle Veranstaltungen', $widget_options );
	}

/* Section 2: */

	public function form( $instance ) {
		$title     = isset( $instance['title'] ) ? esc_attr( $instance['title'] ) : '';
		$number    = isset( $instance['number'] ) ? absint( $instance['number'] ) : 3; ?>
		<p>
			<label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?> <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); ?>" /></label>
		</p>
		<p>
			<label for="<?php echo $this->get_field_id( 'number' ); ?>"><?php _e( 'Anzahl der Veranstaltungen ab heute:' ); ?></label>
			<input class="tiny-text" id="<?php echo $this->get_field_id( 'number' ); ?>" name="<?php echo $this->get_field_name( 'number' ); ?>" type="number" step="1" min="1" value="<?php echo $number; ?>" size="3" />
		</p><?php 
	}

/* Section 3: */

	public function update( $new_instance, $old_instance ) {
		$instance = $old_instance;
		$instance['title']  = sanitize_text_field( $new_instance['title'] );
		$instance['number'] = (int) $new_instance['number'];
		return $instance;
	}

/* Section 4: */

	public function widget( $args, $instance ) {
		$title = ! empty( $instance['title'] ) ? $instance['title'] : 'Veranstaltungen';
		$number = ( ! empty( $instance['number'] ) ) ? absint( $instance['number'] ) : 3;
		$result = new WP_Query( array( 
			'post_type'      => 'wh_event',
			'posts_per_page' => $number,			
			'no_found_rows'  => true,
			'post_status'    => 'publish',
			'meta_key'       => 'eventdate',
	 		'meta_compare'   => '>=',
	 		'meta_value'     => date('c',time()),
	 		'orderby'        => 'meta_value',
	 		'order'          => 'ASC',
		 ), $instance );

		if ( ! $result->have_posts() ) {
			return;
		}

/* Section 5: */

		echo $args['before_widget'];
		if ( $title ) { echo $args['before_title'] . $title . $args['after_title']; }
?>
<ul>
	<?php foreach ( $result->posts as $event ) : ?>
		<?php
		$post_title = get_the_title( $event->ID );
		$title      = ( ! empty( $post_title ) ) ? $post_title : __( '(no title)' );
		$custom     = get_post_custom( $event->ID );
		$location   = ( isset($custom['location'][0] )) ? 'in ' . $custom['location'][0] : '';
		$eventdate  = ( isset($custom['eventdate'][0] )) ? 'am ' . date_i18n(get_option( 'date_format' ), strtotime($custom['eventdate'][0])) : '';
		?>
		<li>
			<a href="<?php the_permalink( $event->ID ); ?>"><strong><?php echo $title ; ?></strong></a><br/><?php echo $eventdate; ?> <?php echo $location; ?>
		</li>
	<?php endforeach; ?>
</ul>
<?php
		echo $args['after_widget'];
	}
}

/* Section 6: */

function register_wh_event_widget() { 
	register_widget( 'wh_event_widget' );
}

add_action( 'widgets_init', 'register_wh_event_widget' );