gistfile1.php

<?php 
    $iterator = new \RecursiveIteratorIterator(
        new \RecursiveDirectoryIterator($dir),
        \RecursiveIteratorIterator::SELF_FIRST
    );

    foreach ($iterator as $file)
        if (!$file->isDir() && $file->getFilename() !== '.' && $file->getFilename() !== '..') {
            $key = pathinfo($file->getPathName(), PATHINFO_DIRNAME);
            if (!isset($data[$key]))
                $data[$key] = [$file->getFilename()];
            else
                $data[$key][] = $file->getFilename();
        }

    return $data;