registeringTermsWP

$cats = [
	'parent',
	'children',
	'grandchildren'
];

$args = [];

for ($i = 0; $i < count($cats); $i++) {

	if (($exists = term_exists($cats[$i], 'category')) === 0 || $exists === false ) {

		if ($i > 0) {

			$parentTerm = term_exists( $cats[$i -1], 'category');

			if ($parentTerm !== 0 && $parentTerm !== false) {

				$args['parent'] =  $parentTerm['term_id'];

			} else {

				throw new Exception('Failed to get parent term');
			}
		}

		$results = wp_insert_term(
			$cats[$i],
			'category',
			$args
		);

		if (is_wp_error($results)) {

			throw new Exception(json_encode($results->errors));
		}
	}
}