hic-et-nunc/site/controllers/index.php

<?php

return function($page) {
    function array_flatten($array) {
      if (!is_array($array)) {
        return false;
      }
      $result = array();
      foreach ($array as $key => $value) {
        if (is_array($value)) {
          $result = array_merge($result, array_flatten($value));
        } else {
          $result = array_merge($result, array($key => $value));
        }
      }
      return $result;
    }

    function getSingleCategories($pages) {
        $categoriesRawFields = $pages->pluck('category'); // return ["A, B, C", "A, B"]
        $splittedCategoriesFields = array_map(function($field) {
            return explode(',', $field);
        }, $categoriesRawFields); // return '[["A", "B", "C",], ["A", "B"]]
        $flattenedCategories = array_flatten($splittedCategoriesFields);
        $trimmed = array_map('trim', $flattenedCategories);
        $filtered = array_filter($trimmed, fn($v) => $v !== '');
        $filters = array_unique($filtered);

        return $filters;
    }

    return [
        'categories' => getSingleCategories($page->children())
    ];
};