Callback validator example

gistfile1.php

<?php

public function getInputFilterSpecification()
{
    return [
        'amount' => [
            'required' => true,
            'validators' => [
                new Validator\GreaterThan([
                    'min' => 0,
                    'inclusive' => false,
                ]),
            ],
        ],
        'privateSpendingAmount' => [
            'required' => false,
            'validators' => [
                new Validator\GreaterThan([
                    'min' => 0,
                    'inclusive' => true,
                ]),
                new Validator\Callback([
                    'callback' => function($value, $context = []) {
                        if ($value && $context['amount'] < $value) {
                            return false;
                        }

                        return true;
                    },
                    'messages' => array(
                        Validator\Callback::INVALID_VALUE => '__invalid-value',
                    ),
                ]),
            ],
        ],       
    ];
}