[WordPress] Rapid Prototyping: Introducing Autoloading

00-autoload.php

<?php
/**
 * Automatically loads the specified file.
 *
 * @package McFarlin\TFP
 */

namespace McFarlin\TFP;

/**
 * Automatically loads the specified file.
 *
 * Examines the fully qualified class name, separates it into components, then creates
 * a string that represents where the file is loaded on disk.
 *
 * @package McFarlin\TFP;
 */
spl_autoload_register(function( $filename ) {

	// First, separate the components of the incoming file.
	$file_path = explode( '\\', $filename );

	// Get the last index of the array. This is the class we're loading.
	if ( isset( $file_path[ count( $file_path ) - 1 ] ) ) {

		$class_file = strtolower(
			$file_path[ count( $file_path ) - 1 ]
		);

		/**
		 * The classname has an underscore, so we need to replace it
		 * with a hyphen for the file name.
		 */
		$class_file = str_ireplace( '_', '-', $class_file );
		$class_file = "class-$class_file.php";
	}

	/**
	 * Find the fully qualified path to the class file by iterating through the $file_path array.
	 * We ignore the first index since it's always the top-level package. The last index is always
	 * the file so we append that at the end.
	 */
	$fully_qualified_path = trailingslashit(
		dirname(
			dirname( __FILE__ )
		)
	);

	/**
	 * We start at the second index of the namespace because our directory
	 * structure does not include 'McFarlin/TRP'.
	 */
	for ( $i = 2; $i < count( $file_path ) - 1; $i++ ) {

		$dir = strtolower( $file_path[ $i ] );
		$fully_qualified_path .= trailingslashit( $dir );
	}
	$fully_qualified_path .= $class_file;

	// Now we include the file.
	if ( file_exists( $fully_qualified_path ) ) {
		include_once( $fully_qualified_path );
	}
});

00-plugin-bootstrap.php

<?php
/**
 * Three Recent Posts
 *
 * @package     TRP
 * @author      Tom McFarlin
 * @copyright   2017 Tom McFarlin
 * @license     MIT
 *
 * @wordpress-plugin
 * Plugin Name: Three Recent Posts
 * Plugin URI:  https://tommcfarlin.com/three-recent-posts/
 * Description: Displays the three mot recent posts in your post editor screen.
 * Version:     1.0.0
 * Author:      Tom McFarlin
 * Author URI:  https://tommcfarlin.com
 * Text Domain: three-recent-posts
 * License:     GPL
 * License URI: http://www.gnu.org/licenses/gpl-3.0.txt
 */

namespace McFarlin\TRP;
use McFarlin\TRP\Display\Meta_Box;

// Import the autoloader.
include_once 'Includes/autoload.php';

add_action( 'add_meta_boxes', __NAMESPACE__ . '\\trp_start' );
/**
 * Starts the plugin.
 */
function trp_start() {

	$meta_box = new Meta_Box( dirname( __FILE__ ) );
	$meta_box->init();
}