Drupal 7, create metatags programality.

scratch.txt

// The type of entity being edited.
$entity_type = 'node';
// The ID of the entity being changed.
$entity_id = 42;
// The revision ID for this entity, leave as NULL to ignore revisioning.
$revision_id = NULL;
// The language code for this entity.
$langcode = LANGUAGE_NONE;
// Load the existing values, will return an empty array if anything found.
$metatags = metatag_metatags_load($entity_type, $entity_id, $revision_id);
// Set the new meta tags. Most meta tags have a 'value' attribute.
$metatags[$langcode]['title']['value'] = 'The title | [site:name]';
$metatags[$langcode]['description']['value'] = 'The description';
// Update the meta tags. This also clears the entity's cache.
metatag_metatags_save($entity_type, $entity_id, $revision_id, $metatags);