Get available term combinations for filtering in WordPress

wp_get_term_filter_options.php

<?php
// Takes an array with a term taxonomy as the item keys and term ids as their values and
// returns the same array but with the values set as term ids that have posts associated
// with the term combination. (PHP7 and above only)
function get_term_filter_options($taxonomies, $query_args = array()) {
  array_walk($taxonomies, function(&$value, $taxonomy) use($taxonomies, $query_args) {
    $posts = null;
    $others = array_filter($taxonomies, function($value, $key) use($taxonomy) {
      return $key !== $taxonomy && !empty($value);
    }, ARRAY_FILTER_USE_BOTH);

    if (!empty($others)) {
      $posts_args = array_merge_recursive($query_args, array(
        'fields'          => 'ids',
        'numberposts'     => -1,
        'tax_query'       => array(
          'relation' => 'AND'
        )
      ));

      foreach($others as $tax => $val) {
        $posts_args['tax_query'][] = array(
          'field'    => 'term_id',
          'taxonomy' => $tax,
          'terms'    => $val
        );
      }

      $posts = get_posts($posts_args);
    }

    $value = get_terms(array(
      'fields'      => 'ids',
      'taxonomy'    => $taxonomy,
      'object_ids'  => $posts
    ));
  });

  return $taxonomies;
}

$filter_options = get_term_filter_options(
  array(
    'Services'            => (int) get_query_var('services'),
    'Market Sectors'      => (int) get_query_var('market-sectors'),
    'Construction Types'  => (int) get_query_var('construction-types')
  ),
  array('post_type' => 'projects')
);

// Returns:
// array(
//  'Services'            => array(16, 19),
//  'Market Sectors'      => array(9, 10),
//  'Construction Types'  => array(3, 4, 6, 5, 7, 29)
// );
?>