Create Drupal 8 taxonomy terms off a YML file. Only two levels are supported.

import-taxonomy.php

<?php

use Symfony\Component\Yaml\Yaml;
use Drupal\taxonomy\Entity\Term;

$config = Yaml::parse(file_get_contents(DRUPAL_ROOT . '/sites/default/files/import.yml'));

$vocabulary = $config['vocabulary'];
foreach($config['terms'] as $term => $kids) {
  $parent_term = Term::create([
    'parent' => [],
    'name' => $term,
    'vid' => $vocabulary,
  ]);
  $parent_term->save();
  if (!is_array($kids)) {
    $kids = [];
  }
  foreach($kids as $kid) {
    Term::create([
      'name' => $kid,
      'parent' => $parent_term->ID(),
      'vid' => $vocabulary,
    ])->save();
  }
}

import.yml

vocabulary: 'test_taxonomy'
terms:
  'term 1':
    - 'term 2'
    - 'term 3'
  'term 4':
    - 'term 5'
    - 'term 6'
  'term 5':