Wordpress Walker 101

class-custom-walker.php

<?php
class CustomWalker extends Walker {
	public $tree_type = 'category';
	public $db_fields = array(
		'parent' => 'parent',
		'id'     => 'term_id',
	);

	public function start_lvl( &$output, $depth = 0, $args = array() ) {
		$output .= "<ul class='children'>\n";
	}
	public function end_lvl( &$output, $depth = 0, $args = array() ) {
		$output .= "</ul>\n";
	}

	public function start_el( &$output, $item, $depth = 0, $args = array(), $current_object_id = 0 ) {
		$output .= "<li>" . $item->name . "\n";
	}

	public function end_el( &$output, $category, $depth = 0, $args = array() ) {
		$output .= "</li>\n";
	}

}

$lists = array(
	(object)[
		'term_id' => '1',
		'name' => 'A',
		'parent' => 0,
	],
	(object)[
		'term_id' => '2',
		'name' => 'B',
		'parent' => 0,
	],
	(object)[
		'term_id' => '3',
		'name' => 'C',
		'parent' => 0,
	],
	(object)[
		'term_id' => '4',
		'name' => 'D',
		'parent' => 1,
	],
	(object)[
		'term_id' => '5',
		'name' => 'E',
		'parent' => 4,
	],
);

$mywalker = new CustomWalker();
echo $mywalker->walk( $lists, 3 );