Drupal 8 Set Message Example

example.php

<?php

// The drupal_set_message() function is being deprecated!
// @see https://api.drupal.org/api/drupal/core%21includes%21bootstrap.inc/function/drupal_set_message/8.5.x
// > Deprecated in Drupal 8.5.0 and will be removed before Drupal 9.0.0.
// > Use \Drupal\Core\Messenger\MessengerInterface::addMessage() instead.

// In some custom code.
\Drupal::messenger()->addMessage('Say something else');

// When trying to print out a simple var.
\Drupal::messenger()->addMessage(print_r($stuff, TRUE));

// In a Drupal 8 Form's submitForm() handler:
$this->messenger()->addMessage($this->t('Hello world.'));

Here's an example using dependency injection: https://www.drupal.org/node/2774931