Attach taxonomy terms to entity programmatically in Drupal 8

in this tuto, I'll show you how to attach taxonomy terms to entity programmatically in Drupal 8.

Add the following code inside a function where you want to the attach taxonomy terms.

<?php

use Drupal\node\Entity\Node;


/**
 * @param $variables
 */
function mymodule_preprocess_page(&$variables)
{
  $node = Node::load('1');
  
  //Attach only one term
  $tid = 3; // The ID of the term to attach.
  $node->set('field_categories', $tid);
  $node->save();
  
  // Attach multiple terms
  $multiple_tids = array(5, 6, 7); // the term IDs must be in an array. Each is Term ID of an existing term.
  $node->set('field_tags', $multiple_tids);
  $node->save();

}