Autoloading for both WordPress classes and PSR-4

autoload.php

<?php
/**
 * Autoload PSR-4 and Wordpress compatible named classes,
 * Require it in very begin of your functions.php or plugin
 *
 * @author Alexandr Shevchenko <ls.shov@gmail.com>
 * @author Thomas J Belknap <tbelknap@holisticnetworking.net>
 */
namespace App; //Change it
/** @var string : autoload root $rootDir */
$rootDir = __DIR__ . '/inc';
(new class
{
    public function __invoke(string $rootDir)
    {
        $rootDir = ($rootDir[-1] === '/') ? $rootDir : $rootDir . '/';
        spl_autoload_register(function ($class) use ($rootDir) {
            array_map(function ($filePath) use ($rootDir) {
                if (false === $filePath) {
                    return;
                }
                if (is_readable("${rootDir}${filePath}.php")) {
                    require_once "${rootDir}${filePath}.php";
                }
            }, [
                $this->wpClassPath($class),
                $this->psr4ClassPath($class),
            ]);
        });
    }
    /**
     * Format the incoming class name to the PSR-4 file naming standard.
     * @param string $class
     * @return bool|string
     */
    private function psr4ClassPath(string $class)
    {
        $parts = $this->splitNameSpace($class);
        if (is_null($parts)) {
            return false;
        }
        $classPath = implode('/', $parts);
        return $classPath;
    }
    /**
     * Format the incoming class name to the WordPress file naming standard.
     * @param string $class
     * @return bool|string
     */
    private function wpClassPath(string $class)
    {
        $parts = $this->splitNameSpace($class);
        if (is_null($parts)) {
            return false;
        }
        $lastPartIndex = count($parts)-1;
        $parts[$lastPartIndex] = 'class-' . $this->wpClassToFile($parts[$lastPartIndex]);
        $classPath = implode('/', $parts);
        return $classPath;
    }
    /**
     * Split and prepare namespace parts
     * @param string $class
     * @return null|array
     */
    private function splitNameSpace(string $class): ?array
    {
        $parts = explode('\\', $class);
        if (__NAMESPACE__ !== $parts[0]) {
            return null;
        }
        
        //Remove root namespace (like App, or your project name)
        unset($parts[0]);
        
        if (empty($parts)) {
            return null;
        }
        
        return array_values($parts);
    }
    /**
     * Translates a WordPress-valid class name to a WordPress-valid file name (e.g. Class_Name - class-name)
     * @param string $name String in camel case format
     * @return string Translated into dashed format
     */
    private function wpClassToFile(string $name): string
    {
        return implode(
            '-',
            explode(
                '_',
                preg_replace_callback(
                    '/[A-Z]*?/',
                    function ($matches) {
                        return strtolower($matches[0]);
                    },
                    $name
                )
            )
        );
    }
})($rootDir);