Programmatic hierarchical book structure

Hierarchical book structure - Drupal 9 sample.php

<?php

// Create seven nodes, put them in a book tree structure.
// A
// |- B
// |  |- C
// |  |- D
// |- E
//    |- F
//    |- G

use Drupal\node\Entity\Node;

$node_a = Node::create([
  'type' => 'page',
  'title' => 'A',
  'book' => [
    'bid' => 'new',
    'pid' => 0,
    'weight' => 0,
  ],
]);
$node_a->save();

$node_b = Node::create([
  'type' => 'page',
  'title' => 'B',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_a->id(),
    'weight' => 0,
  ],
]);

$node_b->save();

$node_c = Node::create([
  'type' => 'page',
  'title' => 'C',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_b->id(),
    'weight' => 0,
  ],
]);
$node_c->save();

$node_d = Node::create([
  'type' => 'page',
  'title' => 'D',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_b->id(),
    'weight' => 1,
  ],
]);
$node_d->save();

$node_e = Node::create([
  'type' => 'page',
  'title' => 'E',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_a->id(),
    'weight' => 1,
  ],
]);
$node_e->save();

$node_f = Node::create([
  'type' => 'page',
  'title' => 'F',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_e->id(),
    'weight' => 0,
  ],
]);
$node_f->save();

$node_g = Node::create([
  'type' => 'page',
  'title' => 'G',
  'book' => [
    'bid' => $node_a->id(),
    'pid' => $node_e->id(),
    'weight' => 1,
  ],
]);
$node_g->save();