WordPress Plugin: A plugin which includes or autoloads classes with namespaces.

coreDIRSEPmodulesDIRSEPclass-my-class.php

<?php
/**
 * An example class in the subfolder: core/modules/core.php
 */
namespace NAMESPACE\Core\Modules;

class My_Class
{
	/**
	 * Saves an instance of the class.
	 *
	 * @var Core
	 */
	private static $instance;
	
	/**
	 * Creates an instance of the class if it does not yet exist.
	 *
	 * @return Core
	 */
	public static function get_instance()
	{
		if ( ! isset( self::$instance ) ) {
			self::$instance = new self();
		}
	
		return self::$instance;
	}
  
  /**
   * Constructor
   */
  public function __construct()
  {
  }  
}

plugin.php

<?php
/**
 * Plugin Name: My Plugin
 * Description: A plugin which includes or autoloads classes with namespaces.
 * Version:     1.0.0
 * Author:      Pixelbart
 * Author URI:  https://pixelbart.de
 */
if ( ! defined( 'ABSPATH' ) ) {
	die();
}

define( 'PLUGIN_FILE', __FILE__ );
define( 'PLUGIN_PATH', plugin_dir_path( __FILE__ ) );

add_action( 'plugins_loaded', array( 'My_Plugin', 'get_instance' ) );

if ( ! class_exists( 'My_Plugin' ) ) {

	class My_Plugin
	{
		/**
		 * Class prefix.
		 *
		 * @var string
		 */
		private $prefix = 'NAMESPACE\\';

		/**
		 * Saves an instance of the class.
		 *
		 * @var My_Plugin
		 */
		private static $instance;
	
		/**
		 * Creates an instance of the class if it does not yet exist.
		 *
		 * @return My_Plugin
		 */
		public static function get_instance()
		{
			if ( ! isset( self::$instance ) ) {
				self::$instance = new self();
			}
	
			return self::$instance;
		}
	
		/**
		 * Constructor.
		 *
		 * @return void
		 */
		private function __construct()
		{
			spl_autoload_register( array( $this, 'autoload' ) );
			
			$this->init();
		}
	
		/**
		 * Includes all classes.
		 *
		 * @param string $class_name The name including namespace of the class.
		 *
		 * @return void
		 */
		public function autoload( $class_name )
		{
			$prefix = $this->prefix;
	
			$len = strlen( $prefix );
	
			if ( 0 !== strncmp( $prefix, $class_name, $len ) ) {
				return;
			}
	
			$relative_class = substr( $class_name, $len );
	
			$path = explode( '\\', strtolower( str_replace( '_', '-', $relative_class ) ) );
			$file = array_pop( $path );
			$file = PLUGIN_PATH . implode( '/', $path ) . '/class-' . $file . '.php';
	
			if ( file_exists( $file ) ) {
				require $file;
			}
		}
	
		/**
		 * Initializes the classes.
		 *
		 * @return void
		 */
		public function init()
		{
			if ( file_exists( PLUGIN_PATH . '/core/vendor/autoload.php' ) ) {
				require_once PLUGIN_PATH . '/core/vendor/autoload.php';
			}

			$classes = [
				'NAMESPACE\Core\Modules\My_Class',
			];

			foreach ( $classes as $class ) :
				call_user_func( array( $class, 'get_instance' ) );
			endforeach;
		}
	}
}