wordpress_category_json_hierarchy.php

<?php

$args = [
    'taxonomy' => 'category',
    'hide_empty' => 0,
    'parent' => 0
];

function _get_child_terms( $items ) {
    foreach ( $items as $item ) {
        $item->children = get_terms( 'category', array( 'child_of' => $item->term_id, 'hide_empty' => 0 ) );
        if ( $item->children ) _get_child_terms( $item->children );
    }
    return $items;
}

$terms = _get_child_terms( get_terms( $args ) );
wp_send_json( $terms );