Extending a custom post type by custom fields through the meta-box mechanism (WordPress 5 verified)

listing-19-2.php

<?php
/**
 * Plugin Name: WH Custom Post Type
 * Version: 0.2
 */
function add_wh_events() {
	$labels = array(
		'name'                  => 'Veranstaltungen',
		'singular_name'         => 'Veranstaltung',
		'add_new'               => 'Erstellen',
		'add_new_item'          => 'Neue Veranstaltung erzeugen',
		'edit_item'             => 'Veranstaltung bearbeiten',
		'new_item'              => 'Neue Veranstaltung',
		'view_item'             => 'Veranstaltung ansehen',
		'view_items'            => 'Veranstaltungen ansehen',
		'search_items'          => 'Nach Veranstaltungen suchen',
		'not_found'             => 'Nichts gefunden',
		'not_found_in_trash'    => 'Nichts im Papierkorb gefunden',
		'all_items'             => 'Alle Veranstaltungen',
		'archives'              => 'Veranstaltungsarchiv',
		'attributes'            => 'Veranstaltungsattribute',
		'insert_into_item'      => 'Einfügen',
		'uploaded_to_this_item' => 'Medien für Veranstaltungen',
		'featured_image'        => 'Veranstaltungs-Flyer',
		'set_featured_image'    => 'Veranstaltungs-Flyer festlegen',
		'remove_featured_image' => 'Veranstaltungs-Flyer entfernen',
		'use_featured_image'    => 'Veranstaltungs-Flyer verwenden',
		'menu_name'             => 'Veranstaltungen',
		'filter_items_list'     => 'Veranstaltungen',
		'items_list_navigation' => 'Veranstaltungen',
		'items_list'            => 'Weitere Veranstaltungen',
		'name_admin_bar'        => 'Veranstaltung',
	);

	$args = array(
		'labels'              => $labels,
		'supports'            => array( 'title', 'editor', 'excerpt', 'thumbnail', 'comments', 'trackbacks', ),
		'taxonomies'          => array( 'category', 'post_tag' ),
		'hierarchical'        => false,
		'public'              => true,
		'show_in_rest'        => true,
		'show_ui'             => true,
		'show_in_menu'        => true,
		'show_in_nav_menus'   => true,
		'show_in_admin_bar'   => true,
		'menu_position'       => 5,
		'can_export'          => false,
		'has_archive'         => true,
		'exclude_from_search' => false,
		'publicly_queryable'  => true,
		'rewrite'             => array('slug' => 'veranstaltung'),
		'capability_type'     => 'page',
	);
	register_post_type( 'wh_event', $args );
}
add_action( 'init', 'add_wh_events', 0 );

function add_wh_event_fields() {
	add_meta_box( 'location', 'Ort', 'add_wh_events_location', 'wh_event', 'side', 'high' );
	add_meta_box( 'eventdate', 'Datum', 'add_wh_events_eventdate', 'wh_event', 'side', 'high' );
}

function add_wh_events_location() {
	global $post;
	$custom = get_post_custom( $post->ID );
	$location = $custom['location'][0];
	echo '<input type="text" name="location" value="' . $location . '" />';
}

function add_wh_events_eventdate( $post ) {
	wp_enqueue_script( 'jquery-ui-datepicker' );
	wp_enqueue_style( 'jquery-ui-style', '//ajax.googleapis.com/ajax/libs/jqueryui/1.12.1/themes/smoothness/jquery-ui.css', true);

	$eventdate = get_post_meta( $post->ID, 'eventdate', true  );
	?>
	<script>
	jQuery(document).ready(function() {
		jQuery('#eventdateid').datepicker({
		dateFormat : 'yy-mm-dd'
		});
	});
	</script>
	<input type="text" name="eventdate" id="eventdateid" value="<?php echo $eventdate; ?>" /></td>
	<?php
}

function save_wh_event_fields( $post_id ) {
	$post_type = get_post_type( $post_id );
    if ( $post_type != 'wh_event' ) {
    	return;
    }
	update_post_meta( $post_id, 'location', $_POST['location'] );
	update_post_meta( $post_id, 'eventdate', $_POST['eventdate'] );
} 

add_action( 'admin_init', 'add_wh_event_fields' );
add_action( 'save_post', 'save_wh_event_fields' );