Set Featured Image to required and add an asterix symbol to title in Wordpress

require-featured-image.php

<?php
/**
 * Change the title of the Featured Image box
 *
 * @package emu
 * @subpackage filters+hooks
 *
 */
function emu_change_featured_image_title() {
	// Replace post_type with the post type you want to have a required Featured Image
	set_transient('thumbnail_post_type', 'post_type');

	// Customize the new Featured Image tile
	$newTitle = 'Featured Image <abbr class="required">*</abbr>';

	// Change the title for the post type above that now require the image
	remove_meta_box( 'postimagediv', get_transient('thumbnail_post_type'), 'side' );
	add_meta_box('postimagediv', __($newTitle), 'post_thumbnail_meta_box', get_transient('thumbnail_post_type'), 'side', 'default');
}
add_action('do_meta_boxes', 'emu_change_featured_image_title');



/**
 * Check to see if a Featured Image has been attached
 *
 * @package emu
 * @subpackage filters+hooks
 * @param int $post_id 	Post ID
 *
 */
function emu_check_featured_image($post_id) {
	// If post type doesn't match current post type then just keep going
	if( get_post_type($post_id) != get_transient('thumbnail_post_type') ) {
		delete_transient('thumbnail_post_type');
		return;
	}

	if ( !has_post_thumbnail( $post_id ) && (get_post_status( $post_id ) == 'publish' || get_post_status( $post_id ) == 'auto-draft') ) {
		// set a transient to show the users an admin message
		set_transient('has_post_thumbnail', 'no');
		// unhook this function so it doesn't loop infinitely
		remove_action('save_post', 'emu_check_featured_image');
		// update the post set it to draft
		wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));

		add_action('save_post', 'emu_check_featured_image');
	} else {
		delete_transient('thumbnail_post_type');
		delete_transient('has_post_thumbnail');
	}
}
add_action('save_post', 'emu_check_featured_image');



/**
 * Check to see if a Featured Image has been attached, if it hasn't then throw
 * an error message
 *
 * @package emu
 * @subpackage filters+hooks
 *
 */
function emu_featured_image_error() {
	// check if the transient is set, and display the error message
	if ( get_transient( 'has_post_thumbnail' ) == 'no' ) {
		echo '<div id="message" class="error"><p><strong>You must select a Featured Image. Your post has been saved but it not published.</strong></p></div>';
		delete_transient( 'has_post_thumbnail' );
	}
}
add_action('admin_notices', 'emu_featured_image_error');