<?php
// getTerms.php on remote WordPress site
require_once $_SERVER['DOCUMENT_ROOT'] . '/wp-load.php';
require_once $_SERVER['DOCUMENT_ROOT'] . '/wp-admin/includes/admin.php';

// Get the terms
$terms = get_terms([
    'taxonomy'   => 'category',
    'hide_empty' => false,
]);

// Return as JSON
header('Content-Type: application/json');
echo json_encode($terms);
exit;