doc.silverstripe.org/sapphire/docs/en/reference/versioned.md

Versioned

The Versioned class is a [api:DataObject] that adds versioning and staging capabilities to the objects.

Trapping the publication event

Sometimes, you'll want to do something whenever a particular kind of page is published. This example sends an email whenever a blog entry has been published.

SilverStripe 2.3

:::php
class Page extends SiteTree {
  // ...
  function publish($fromStage, $toStage, $createNewVersion = false) {
    mail("sam@silverstripe.com", "Blog published", "The blog has been published");
    return $this->extension_instances['Versioned']->publish($fromStage, $toStage, $createNewVersion);
  }
}

SilverStripe 2.4

:::php
class Page extends SiteTree {
  // ...
  function onAfterPublish() {
    mail("sam@silverstripe.com", "Blog published", "The blog has been published");
    parent::onAfterPublish();
  }
}