WordPressでautoloadを作成する

namespace Sample\Plugin;

abstract class Plugin_Base {
	public $slug;

	public $dir_path;

	public $dir_url;
  
  // 読み込み対象のフォルダ
	protected $autoload_class_dir = 'includes';

	protected $autoload_matches_cache = [];

  public function __construct() {
    $location       = $this->locate_plugin();
		$this->slug     = $location['dir_basename'];
		$this->dir_path = $location['dir_path'];
		$this->dir_url  = $location['dir_url'];
		spl_autoload_register( [ $this, 'autoload' ] );
  }

	public function get_object_reflection() {
		static $reflection;
		if ( empty( $reflection ) ) {
			// @codeCoverageIgnoreStart
			$reflection = new \ReflectionObject( $this );
			// @codeCoverageIgnoreEnd
		}

		return $reflection;
	}

	public function relative_path( $path, $start, $sep ) {
		$path = explode( $sep, untrailingslashit( $path ) );
		if ( count( $path ) > 0 ) {
			foreach ( $path as $p ) {
				array_shift( $path );
				if ( $p === $start ) {
					break;
				}
			}
		}

		return implode( $sep, $path );
	}

	public function locate_plugin() {
		$file_name = $this->get_object_reflection()->getFileName();

		// Windows compat.
		if ( '/' !== \DIRECTORY_SEPARATOR ) {
			// @codeCoverageIgnoreStart
			$file_name = str_replace( \DIRECTORY_SEPARATOR, '/', $file_name );
			// @codeCoverageIgnoreEnd
		}

		$plugin_dir  = dirname( dirname( $file_name ) );
		$plugin_path = $this->relative_path( $plugin_dir, basename( content_url() ), '/' );

		$dir_url      = content_url( trailingslashit( $plugin_path ) );
		$dir_path     = $plugin_dir;
		$dir_basename = basename( $plugin_dir );

		return compact( 'dir_url', 'dir_path', 'dir_basename' );
	}

	public function autoload( $class ) {
		if ( ! isset( $this->autoload_matches_cache[ $class ] ) ) {
			if ( ! preg_match( '/^(?P<namespace>.+)\\\\(?P<class>[^\\\\]+)$/', $class, $matches ) ) {
				$matches = false;
			}

			$this->autoload_matches_cache[ $class ] = $matches;
		} else {
			$matches = $this->autoload_matches_cache[ $class ];
		}

		if ( empty( $matches ) ) {
			return;
		}

		$namespace = $this->get_object_reflection()->getNamespaceName();

		if ( strpos( $matches['namespace'], $namespace ) === false ) {
			return;
		}

		$class_name = $matches['class'];
		$class_path = \trailingslashit( $this->dir_path );

		if ( $this->autoload_class_dir ) {
			$class_path .= \trailingslashit( $this->autoload_class_dir );

			$sub_path = str_replace( $namespace . '\\', '', $matches['namespace'] );
			if ( ! empty( $sub_path ) && 'Sample\Plugin' !== $sub_path ) {
				$class_path .= str_replace( '\\-', '/', strtolower( preg_replace( '/(?<!^)([A-Z])/', '-\\1', $sub_path ) ) . '/' );
			}
		}

		$class_path .= sprintf( 'class-%s.php', strtolower( str_replace( '_', '-', $class_name ) ) );

		if ( is_readable( $class_path ) ) {
			require_once $class_path;
		}
	}
}

namespace Sample\Plugin;

class Plugin extends Plugin_Base {

    /**
     * コンストラクタ
     */
    public function init() {
        new Sample();
        new Test();
        new Example();
        add_action( 'wp_enqueue_scripts', [ $this, ... ] );
    }
}