Drupal 7 Snippet to create taxonomy term by name, if it doesn't exist yet and return the term object anyways.

create_term.module

/**
 * Helper function to create a term by name, if it doesn't exist yet.
 *
 * @param $term_name string
 *   Human readable term name.
 * @param $vocabulary_name string
 *   Vocabulary machine name.
 * @param $weight integer.
 *   Term weight.
 *
 * @return object
 */
function _create_term($term_name, $vocabulary_name, $weight) {

  $term = taxonomy_get_term_by_name($term_name, $vocabulary_name);

  if (!empty($term)) {
    return $term;
  }

  $vocab = taxonomy_vocabulary_machine_name_load($vocabulary_name);
  $term = new stdClass();
  $term->name = $term_name;
  $term->vid = $vocab->vid;
  $term->weight = $weight;
  taxonomy_term_save($term);

  return $term;
}