gistfile1.php

<?php

/**
 * @param $path
 * @param $extension
 *
 * @return array
 */
function getFilesInPathWithExtension($path, $extension)
{
  $directoryIterator = new RecursiveDirectoryIterator($path);
  $recursiveIterator = new RecursiveIteratorIterator($directoryIterator);

  $files = [];

  foreach ($recursiveIterator as $item) {
    if ($item->isFile() and $item->getExtension() === $extension) {
      $files[] = $item;
    }

    if ($item->isDir() and $item->getFilename() !== "." and $item->getFilename() !== "..") {
      $files = array_merge(
        $files, getFilesInPathWithExtension($item->getPathName(), $extension)
      );
    }
  }

  return $files;
}

note that the Symfony Finder component is actually just a builder for the SPL-based code (its IteratorAggregate implementation will build the SPL iterator stack corresponding to the conditions added in the finder)