admin-id.php

<?php
/*
Plugin Name: Add Post / Page ID Column
Description: Add ID column on Post and Page listing in the ClassicPress admin area.
Author: Matt Robinson
Version: 0.1
*/

if ( is_admin() ) {
	$add_admin_id = new Add_Admin_Id();
}

class Add_Admin_Id {
	public function __construct() {
		add_action( 'load-edit.php', array( $this, 'add_id_init' ) );
	}

	public function add_id_init() {
		$screen = get_current_screen();

		if ( ! isset( $screen->post_type ) || ! in_array( $screen->post_type, array( 'post', 'page' ), true ) ) {
			return;
		}

		add_filter( "manage_{$screen->id}_columns", array( $this, 'add_id_col' ) );
		add_action( 'admin_head', array( $this, 'add_id_style' ) );
		add_action( "manage_{$screen->post_type}_posts_custom_column", array( $this, 'add_id_data_cb' ), 10, 2 );
		add_filter( "manage_{$screen->id}_sortable_columns", array( $this, 'add_id_data_sortable' ) );
	}


	public function add_id_col( $cols ) {
		$cols = array_reverse( $cols, true );
		$cb   = array_pop( $cols );

		$cols['id'] = 'ID';
		$cols['cb'] = 'cb';

		return array_reverse( $cols, true );
	}

	public function add_id_style() {
		echo '<style>.wp-list-table .column-id { width: 5%; }</style>';
	}

	public function add_id_data_cb( $col, $post_id ) {
		if ( 'id' === $col ) {
			echo esc_html( $post_id );
		}
	}

	public function add_id_data_sortable( $cols ) {
		$cols['id'] = 'template';
		return $cols;
	}
}