Zend Framework 2 Form Attaching a custom validator to an element that allows nulls.

MyCustomValidator.php

<?php
namespace Application\Validator;

use Zend\Validator\AbstractValidator;

class MyCustomValidator extends AbstractValidator
{
    const CONDITIONAL_EMPTY = 'conditionally_empty';
    const SOMETHING_BAD = 'something_bad';

    protected $messageTemplates = array(
        self::CONDITIONAL_EMPTY => 'The value is required if you select something else',
        self::SOMETHING_BAD => 'Something bad happened',
    );

    // bring in the context in the 2nd argument
    // but default to null to match the parent signature
    public function isValid($value, $context = null)
    {
        $this->setValue($value);

        // here's the allow empty bit,
        // say if the user didn't select 'something_else', this field isn't required.
        if ('' == $context['something_else']) {
            return true;
        }
        
        // do your extra validation here, since 'something_else' had a value...
        $error = false;

        if ($error) {
            $this->error(self::SOMETHING_BAD);
            return false;
        }

        return true;
    }
}

MyFilter.php

<?php
namespace Application\Filter;

use Zend\InputFilter\InputFilter;
use Zend\Validator\NotEmpty;
use Application\Validator\MyCustomValidator;

class MyCustomFilter extends InputFilter
{
    public function __construct()
    {
        // this is the field that may be empty
        $this->add(array(
            'name' => 'the_field',
            // don't use these here; leave them out
            // 'required' => true,
            // 'allow_empty' => true,
            'filters' => array(
              // filters go here
            ),
            // no 'validators' array
        ));

        $myValidator = new MyCustomValidator();
        $myValidator->setMessages(
            array(
              // your messages
              'conditionally_empty' => 'the_field cannot be empty you select something_else'
            )
        );

        $theField = $this->get('the_field');
        $theField->getValidatorChain()
            // reattach the NotEmpty filter with NotEmpty::NULL
            ->attach(new NotEmpty(NotEmpty::NULL))
            // attach your custom validator
            // second argument is 'break chain on error'
            ->attach($myValidator, true);
            
        /*
        // other fields in this filter
        $this->add(array(
            'name' => 'another_field',
        ));
        // etc.
        // */
    }
}