Drupal: Set module implementation weight after that of another module

my_module.php

<?php

/**
 * Implements hook_module_implements_alter().
 */
function my_module_module_implements_alter(&$implementations, $hook) {
  if ($hook === 'form_alter') {
    // Implement after simplesamlphp_auth.
    if (isset($implementations['simplesamlphp_auth'])) {
      $my_module = ['my_module' => $implementations['my_module']];
      unset($implementations['nyun_core']);
      $offset = array_search('simplesamlphp_auth', array_keys($implementations)) + 1;
      $implementations = array_slice($implementations, 0, $offset, TRUE) +
        $my_module +
        array_slice($implementations, $offset, NULL, TRUE);
    }
  }
}