这是一个可以像WordPress一样使用的函数get_categories 但返回层次结构数组。为此，有一个child_categories 属性添加到每个WP_TERM 对象：

function get_categories_hierarchical( $args = array() ) {

    if( !isset( $args[ \'parent\' ] ) ) $args[ \'parent\' ] = 0;

    $categories = get_categories( $args );

    foreach( $categories as $key => $category ):

        $args[ \'parent\' ] = $category->term_id;

        $categories[ $key ]->child_categories = get_categories_hierarchical( $args );

    endforeach;

    return $categories;

}

例如，可以使用此函数（从类别层次结构的前两个级别构建一行HTML select输入）：

<?php if( $categories = get_categories_hierarchical() ): foreach( $categories as $category ): ?>

    <label for="filter_cat_<?php echo $category->term_id; ?>"><?php echo $category->name; ?></label>

    <select name="filter_cat_<?php echo $category->term_id; ?>" id="filter_cat_<?php echo $category->term_id; ?>">

        <?php if( $category->child_categories ) foreach( $category->child_categories as $subcategory ): ?>

            <option value="<?php echo $subcategory->term_id; ?>"<?php if( $_GET[ \'filter_cat_\' . $category->term_id ] == $subcategory->term_id ) echo \' selected\'; ?>><?php echo $subcategory->name; ?></option>

        <?php endforeach; ?>

    </select>

<?php endforeach; endif; ?>

也请查看wp_dropdown_categories 和wp_list_categories wich已经可以做您需要的事情，而不需要额外的功能。