Helper function to Sort an array of objects by multiple keys.

_SortByOrder.php

<?php

/**
 * Takes an array of objects, and sorts them by one or more keys.
 * @param  array &$list      A one-dimensional array of objects
 * @param  array $keys       Array of strings corresponding to the keys which we want to sort on.
 * @param  array $keys_order Optional array of strings for the sort order. Valid values are 'asc' or 'desc'.
 *                           Placement corresponds to our $keys array (So $keys = ['a', 'b'] and $keys_order - ['desc', 'asc']
 *                           Will sort $list by key 'a' first in descending order, then by key 'b' in ascdening order).
 *                           Default sort order will be SORT_ASC.
 * @param  array $key_flags  Optional array of strings for the sort flags. 
 *                           Defaults to SORT_REGULAR.
 * @return bool              Returns false if $list or $keys is unset and null, or if array_multisort fails.
 *                           Returns true if multisort is successful.
 */
function _sortByOrder(&$list, $keys, $keys_order = null, $key_flags = null) {
  if (!isset($list) || !isset($keys)) {
    return false;
  }

  /**
   * Array of "cols" within our list.
   *
   * So...
   * 
   * $list = [
   *   ['a' => 0, 'b' => 1],
   *   ['a' => 2, 'b' => 3],
   *   ['a' => 4, 'b' => 5]
   * ];
   *      
   * will yield:
   * 
   * $keys_array = [
   *   'a' => [0, 2, 4],
   *   'b' => [1, 3, 5]
   * ];
   *
   */
  $keys_array = array();

  foreach ($list as $i => $item) {
    foreach($keys as $k => $key) {
      $keys_array[$key][$i]  = $item[$key];

      // Since we're looping through our $keys, check for sort_order
      if (!isset($keys_array[$key]['sort_order'])) {
        // Use SORT_ASC if none are present
        $keys_array[$key]['sort_order'] = ((isset($keys_order[$k]) && $keys_order[$k] === 'desc') ? SORT_DESC : SORT_ASC);
      }

      // And check for sort_flags
      if (!isset($keys_array[$key]['sort_flags'])) {
        // Use SORT_REGULAR if no value is present
        switch ((isset($key_flags[$k]) ? $key_flags[$k] : 'regular')) {
          case 'numeric':
            $sort_flag = SORT_NUMERIC;
            break;
          case 'string':
            $sort_flag = SORT_STRING;
            break;
          case 'locale_string':
            $sort_flag = SORT_LOCALE_STRING;
            break;
          case 'natural':
            $sort_flag = SORT_NATURAL;
            break;
          case 'flag_case':
            $sort_flag = SORT_FLAG_CASE;
            break;
          case 'regular':
          default:
            $sort_flag = SORT_REGULAR;
            break;
        }
        $keys_array[$key]['sort_flags'] = $sort_flag;

        unset($sort_flag);
      }
    }
  }

  // Build list of arguments to be unpacked in our array_multisort call
  $arg_array = array();

  foreach ($keys_array as $key_name => $arg) {
    $sort_order = $arg['sort_order'];
    unset($arg['sort_order']);

    $sort_flag = $arg['sort_flags'];
    unset($arg['sort_flags']);

    $arg_array[] = $arg;
    $arg_array[] = $sort_order;
    $arg_array[] = $sort_flag;
  }

  $arg_array[] = &$list;

  // Sort the columns and modify our $list in place
  // Requires PHP 5.6
  return array_multisort(...$arg_array);
}