Refactor code for creation specific array from directory structure

I've already asked about array creation with specific structure from directory structure (Create array from directory structure). Briefly. Now I have next directory structure:

  collection
      |
      |
      ---buildings
      |     |
      |     |
      |     |    
      |     ---2.php
      |     |
      |     ---4.php
      |
      |
      ---trees
          |
          |
          ---1.php
          |     
          ---2.php
          |
          ---3.php

I neet to iterate this directory structure and create next array:

array(
  0 => array('category' => 'trees',
         'file' => '3.jpg'
  ),
  1 => array('category' => 'trees', 
         'file' => '2.php'
  ),
  2 => array('category' => 'trees', 
         'file' => '1.jpg'
  ),
  3 => array('category' => 'buildings', 
         'file' => '2.jpg'
  ),
  4 => array('category' => 'buildings',
        'file' => '4.php'
  ))

So I've implemented it using RecursiveIterstorIterator and RecursiveDirectoryIterator. There is my code:

$path = __DIR__ . '/collection/';
$dir  = new RecursiveDirectoryIterator($path,     RecursiveDirectoryIterator::SKIP_DOTS);
$files = new RecursiveIteratorIterator($dir,     RecursiveIteratorIterator::SELF_FIRST);
$index = 0;
$paths = [];

foreach ($files as $file) {

    $category = $paths[$index-1]['category'];

    if (strpos($file->getPathname(), $paths[$index-1]['category'])) {
        $paths[$index]['category'] = $paths[$index-1]['category'];
        $paths[$index]['file'] = $file->getFilename();
    }

    else {
        if ($paths[$index]['category']) {
            $paths[$index]['category'];
        } else {
            if ($file->isDir()) {
                $paths[$index]['category'] = $file->getFilename();
            }

            $files->next();

            $file = $files->current();

            if ($file->isFile()) {
                $paths[$index]['file'] = $file->getFilename();
            }
        }
    }

    $index++;
}

var_dump($paths);

But I think my code is 'dirty' and can use another functionality of RecursiveIteratorIterator and RecursiveDirectoryIterator avoid this awful if...else nesting. Also I think I didn't consider all usecases. So please could you please help me to refactor my code! Thanks!