private static function create_terms()
{
$taxonomies = array('product_type' => array('simple', 'grouped', 'variable', 'external'));
foreach ($taxonomies as $taxonomy => $terms) {
foreach ($terms as $term) {
if (!get_term_by('slug', sanitize_title($term), $taxonomy)) {
wp_insert_term($term, $taxonomy);
}
}
}
}