A Drupal 7 helper function that takes a taxonomy term name and returns a corresponding term ID – even if it has to create the term first. Also supports adding Parent -> Child terms.

gistfile1.php

/**
 * Helper function to link term names to taxonomy terms, and create new terms
 * on-the-fly when necessary.
 *
 * @param $name
 *  - The name of the taxonomy term.
 * @param $vocabulary_name
 *  - The machine name of the vocabulary where the term should be stored.
 * @param $parent_name
 *  - Optionally the name of the parent term.
 * @return
 *  - The term id (tid).
 */
function _mymodule_get_taxo_term($name, $vocabulary_name, $parent_name = '') {
  // See if the term exists in the chosen vocabulary and return the tid. 
  // If so, disregard the parent name to avoid duplicates.
  if ($possibilities = taxonomy_get_term_by_name($name, $vocabulary_name)) {
    $term = array_pop($possibilities);
    return $term->tid;
  }
  // Otherwise, create a new term and return the tid.
  elseif ($vocabulary = taxonomy_vocabulary_machine_name_load($vocabulary_name)) {
    // Handle parent term first, using a recursive call.
    if (!empty($parent_name)) {
      $parent_tid = _mymodule_get_taxo_term($parent_name, $vocabulary_name);
    }
    $term = (object) array(
      'vid' => $vocabulary->vid,
      'name' => trim($name),
    );
    if ($parent_tid) {
      $term->parent = $parent_tid;
    }
    taxonomy_term_save($term);

    return $term->tid;
  }

  return false;
}