Recursively get WordPress taxonomy and its children, adding to term object as sub-array

functions.php

<?php


if( ! function_exists( 'smyles_get_taxonomy_hierarchy' ) ){

	/**
	 * Recursively get taxonomy and its children
	 *
	 * @param string $taxonomy
	 * @param int    $parent    Parent term ID (0 for top level)
	 * @param array  $args      Array of arguments to pass to get_terms (to override default)
	 *
	 * @return array
	 */
	function smyles_get_taxonomy_hierarchy( $taxonomy, $parent = 0, $args = array( 'hide_empty' => false ) ) {

		$defaults = array(
			'parent' => $parent,
			'hide_empty' => false
		);

		$r = wp_parse_args( $args, $defaults );

		// get all direct decendants of the $parent
		$terms = get_terms( $taxonomy, $r );

		// prepare a new array.  these are the children of $parent
		// we'll ultimately copy all the $terms into this new array, but only after they
		// find their own children
		$children = array();
		// go through all the direct decendants of $parent, and gather their children
		foreach ( $terms as $term ) {
			// recurse to get the direct decendants of "this" term
			$term->children = smyles_get_taxonomy_hierarchy( $taxonomy, $term->term_id );
			// add the term to our new array
			$children[ $term->term_id ] = $term;
		}

		// send the results back to the caller
		return $children;
	}

}

Original code base based off of https://www.daggerhart.com/wordpress-get-taxonomy-hierarchy-including-children/