Trap Reserved Username For ZfcUser module

Module.php

<?php
namespace TrapReservedUsernameForZfcUser;

use Zend\Mvc\MvcEvent;

use Zend\Validator\ValidatorChain;
use Zend\Validator\Callback;
use ZfcUser\Validator\NoRecordExists;

class Module
{
    public function onBootstrap(MvcEvent $e)
    {
        $sm = $e->getApplication()->getServiceManager();
        $form = $sm->get('zfcuser_register_form');
        $inputFilter = $form->getInputFilter();
        $this->insertReservedUsernameValidatorToInput($inputFilter->get('username'));
    }

    private function insertReservedUsernameValidatorToInput($input)
    {
        $usernameValidator = function($name) {
            // borrowed from 
            // http://qiita.com/phimcall/items/4c559b70f70ea7f1953b
            $list = 'index, home, top, help, about, security, contact, connect, support, faq, form, mail, update, mobile, phone, portal';
            return !in_array($name, explode(", ", $list));
        };

        $validators = $input->getValidatorChain()->getValidators();
        $validators = $this->insertCallbackValidatorBeforeNoRecordExists($validators, $usernameValidator);
        $validatorChain = new ValidatorChain();
        foreach ($validators as $validator) {
            $validatorChain->attach($validator['instance'], $validator['breakChainOnFailure']);
        }
     
        $input->setValidatorChain($validatorChain);   
    }

    private function insertCallbackValidatorBeforeNoRecordExists($validators, $callback)
    {
        $newValidators = [];
        $newValidatorCount = count($validators) + 1;
        for ($i = 0, $k = 0; $i < $newValidatorCount; $i++, $k++) {
            if ($validators[$k]['instance'] instanceof NoRecordExists) {
                $newValidators[$i]['instance'] = new Callback($callback);
                $newValidators[$i]['breakChainOnFailure'] = false;
                ++$i;
            }
            $newValidators[$i] = $validators[$k];
        }
    
        return $newValidators;
    }
}