autoload.php

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

/**
 * Anonymous function that registers a custom autoloader
 */
return function ( $prefix, $baseDir ) {
	spl_autoload_register(
		function ( $class ) use ( $prefix, $baseDir ) {
			// does the class use the namespace prefix?
			$len = strlen( $prefix );
			if ( strncmp( $prefix, $class, $len ) !== 0 ) {
				// no, move to the next registered autoloader
				return;
			}

			// get the relative class name
			$relative_class = substr( $class, $len );

			// replace the namespace prefix with the base directory, replace namespace
			// separators with directory separators in the relative class name, append
			// with .php
			$file = $baseDir . str_replace( '\\', '/', $relative_class ) . '.php';

			// if the file exists, require it
			if ( file_exists( $file ) ) {
				require $file;
			}
		}
	);
};