Export configuration in Drupal 8

export.php

<?php
use Drupal\Core\Config\FileStorage;

// Empty destination dir and then write all .yml files there.
$source_storage = Drupal::service('config.storage');
$destination_storage = new FileStorage('/tmp/config');

// Export configuration
foreach ($source_storage->listAll() as $name) {
  $destination_storage->write($name, $source_storage->read($name));
}

// Export configuration collections.
foreach (\Drupal::service('config.storage')->getAllCollectionNames() as $collection) {
  $source_storage = $source_storage->createCollection($collection);
  $destination_storage = $destination_storage->createCollection($collection);
  foreach ($source_storage->listAll() as $name) {
   $destination_storage->write($name, $source_storage->read($name));
  }
}