classes.php

<?php

// the classes

abstract class File {
  // visitor pattern
  function accept (TreeRenderer $renderer) {
    $renderer->render($this);
  }
}

class Folder extends File {
  public $files = [];
  public $name = "Folder";
  
}

class Image extends File {
  public $url = "foo.jpg";
}

TreeRenderer.php

<?php

// the renderers - uses

class TreeRenderer {
  
  public function render (File $file) {
    // we'll use string concats, but you could also use a node tree and transform to XML
    $result = "";
    if ($file instanceof Folder) {
        $result .= "<li>";
        $result .= e($file->name);
        $result .= "<ul>";
        // fun fact - using the visitor pattern we could loop through children in Folder::accept,
        // but we can't because we needed to render folders _around_ the files, not just before/after
        foreach ($file->files as $subFile) {
          $result .= $this->render($subFile);
        }
        $result .= "</ul></li>";
    } elseif ($file instanceof Image) {
        $result .= "<li>";
        $result .= '<img src="'.url($file->url).'" />'
        $result .= "</li>";
    }
  }
  
}

TreeRendererHelper.php

<?php

// usage example - you can do this wherever
class TreeRendererHelper {
  
  public function draw ($file) {
    
    $renderer = new TreeRenderer;
    $html = $renderer->render($file);
    return $html;
    
  }
  
}