$iterator = new RecursiveIteratorIterator(
  new RecursiveDirectoryIterator($dirpath),
  RecursiveIteratorIterator::SELF_FIRST
);
$iterator->setFlags(RecursiveDirectoryIterator::SKIP_DOTS);

foreach($iterator as $path) {
  // Do something
}

$iterator = new RecursiveIteratorIterator(
  new RecursiveDirectoryIterator([$dirpath1, $dirpath2]),
  RecursiveIteratorIterator::SELF_FIRST
);
$iterator->setFlags(RecursiveDirectoryIterator::SKIP_DOTS);

foreach($iterator as $path) {
  // Do something
}