Add a custom, sortable column to a WordPress post type admin table

wp-sortable-column.php

<?php
class HWID_Post_Sortable {
  
  static $post_type_slug = 'post';

	static function init() {

		add_action( 'manage_' . self::$post_type_slug . '_posts_custom_column', array(
			'HWID_Post_Sortable',
			'sort_column_display',
		), 10, 2 );

		add_filter( 'manage_' . self::$post_type_slug . '_posts_columns', array(
			'HWID_Post_Sortable',
			'custom_columns',
		) );

		add_filter( 'manage_edit-' . self::$post_type_slug . '_sortable_columns', array(
			'HWID_Post_Sortable',
			'custom_sortable_column',
		) );

		add_action( 'pre_get_posts', array(
			'HWID_Post_Sortable',
			'custom_sort_order',
		) );

	}

	/**
	 * Populate the display value of the Display Order column in the admin table
	 *
	 * @param string $column
	 * @param string|int $post_id
	 */
	static function sort_column_display( $column, $post_id ) {
		switch ( $column ) {
			case 'display_order':
				if ( ! $display_order = get_post_meta( $post_id, 'display_order', true ) ) {
					$display_order = 9999;
				}

				echo $display_order;
				break;
		}
	}

	/**
	 * Make the display order column sortable
	 *
	 * @param array $columns
	 *
	 * @return array
	 */
	static function custom_sortable_column( $columns ) {

		$columns['display_order'] = 'display_order';

		return $columns;

	}

	/**
	 * Add Display Order to the custom columns
	 *
	 * @param array $columns
	 *
	 * @return array
	 */
	static function custom_columns( $columns ) {

		return self::add_column_array( $columns, array(
			'display_order' => __( 'Display Order', 'hwid' ),
		) );

	}

	/**
	 * Sort the admin columns by display order if specified
	 *
	 * @param \WP_Query $wp_query
	 */
	static function custom_sort_order( $wp_query ) {

		if ( ! is_admin() ) {
			return;
		}
		if ( ! $wp_query->is_main_query() ) {
			return;
		}
		if ( ! $wp_query->get('post_type' ) === self::$post_type_slug ) {
			return;
		}

		if ( ! $orderby = $wp_query->get( 'orderby' ) ) {
			return;
		}

		switch ( $orderby ) {
			case 'display_order':
				$wp_query->set( 'orderby', 'meta_value_num' );
				$wp_query->set( 'meta_key', 'display_order' );
				$wp_query->set( 'order', $wp_query->get( 'order' ) );
				break;
		}

	}

	/**
	 * Add a new admin column to the second to last position
	 *
	 * @param array $columns - Existing columns
	 * @param array $new_column - New column to add
	 *
	 * @return array
	 */
	static function add_column_array( $columns, $new_column ) {

		$first_chunk = array_slice( $columns, 0, count( $columns ) - 1 );
		$second_chunk = array_slice( $columns, -1 );

		return array_merge( $first_chunk, $new_column, $second_chunk );
	}
}