category对象存储父ID，如下所示：$category->parent

如果没有父项，则等于0.

这样，就可以根据的值创建一个等于父类或子父类的变量$category->parent:

$categories = get_categories();
if ( ! is_wp_error( $categories ) ) {
    foreach ( $categories as $category ) {
        $htmlclass = ( $category->parent === 0 ) ? "level-0" : "level-1";
        echo \'<li class="\' . esc_attr( $htmlclass ).\'">\';
        echo \'<a href="\' . esc_url( get_category_link( $category->term_id ) ) . \'">\';
        echo esc_html( $category->name );
        echo \'</a></li>\';
    }
}

受到@Sabry回答的极大启发。

考虑了术语嵌套深度。此外，它适用于所有分类法。

<?php
$taxonomy = \'category\'; // your requred

$terms = get_terms(
    array(
        \'taxonomy\' => $taxonomy,
    )
);

if ( ! is_wp_error( $terms ) ) {
    foreach ( $terms as $term ) {
        $term_ancestors = get_ancestors( $term->term_id, $taxonomy, \'taxonomy\' );

        $term_nesting_depth = count( $term_ancestors );

        $htmlclass = \'level-\' . $term_nesting_depth;

        echo \'<li class="\' . esc_attr( $htmlclass ) . \'">\';
        echo \'<a href="\' . esc_url( get_term_link( $term, $taxonomy ) ) . \'">\';
        echo esc_html( $term->name );
        echo \'</a></li>\';
    }
}