[WordPress] Using an interface to help build meta box-based functionality.

00-interface-meta-box.php

<?php

interface Interface_Meta_Box {

	public function add();
	public function display();
	public function save();

}

01-example-meta-box.php

<?php

class Example_Meta_Box implements Interface_Meta_Box {

	public function init() {
		add_action( 'add_meta_boxes', array( $this, 'add' ) );
	}

	public function add() {

		add_meta_box(
			'example_meta_box',
			'Example Meta Box',
			array( $this, 'display' ),
			'post'
		);

	}

	public function display() {
		echo "This is an example meta box.";
	}

	public function save() {

		/* This is where the save functionality goes.
		 *
		 * It should check to make sure the current user has permission
		 * to save, and then should update whatever information - such as post meta -
		 * that's relevant to the current post and the data presented in the meta box,
		 * if any.
		 */
	}

}

03-meta-box.php

<?php
/**
 * Plugin Name:       Meta Box Example
 * Description:       Demonstrates the use of Meta Boxes, interfaces, and more in WordPress.
 * Version:           1.0.0
 * Author:            Tom McFarlin
 * Author URI:        https://tommcfarlin.com/
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

/**
 * The interface that defines the functions for the meta box.
 */
include_once( 'interface-meta-box.php' );

/**
 * The class that implements the interface and defines the meta  box.
 */
include_once( 'class-example-meta-box.php' );

/**
 * Instantiates the main class and initializes the plugin.
 */
function mbe_start() {

	$plugin = new Example_Meta_Box();
	$plugin->init();

}
mbe_start();