AbstractValidator.php

<?php
class SpecializedValidator extends AbstractValidator
{
    private $somethingToCheck;

    public function __construct($somethingToCheck)
    {
        parent::__construct([$this, 'validate']); // sic!
        $this->somethingToCheck = $somethingToCheck;
    }

    /**
     * @return bool|string
     */
    protected function validate()
    {
        if ($this->somethingToCheck === 'foo') {
            return true;
        }

        return 'expected foo!';
    }
}

 // sic!
class AbstractValidator
{
    /**
     * @var callable
     */
    private $callback;

    public function __construct(callable $callable, $customErrorCode = null) {
        // store its callback
        $this->callback = $callable;

        // assure its callback
        if (!is_callable($callable)) {
            throw new \Exception('First argument has to be a valid callback!');
        }
    }

    public function isValid($value, &$messages = array())
    {
        $message = call_user_func_array($this->callback, func_get_args());

        // assure validity
        if ($message === true) {
            return true;
        }

        // set as invalid
        $messages[] = $message;

        // return
        return false;
    }
}

 // let's see how this works
$msg = [];
$v = new SpecializedValidator('foo');
var_dump($v->isValid('this value is completely irrelevant', $msg));
var_dump($msg);

$v = new SpecializedValidator('bar');
var_dump($v->isValid('this value is completely irrelevant', $msg));
var_dump($msg);

A WTF I found in some code today... abstracted and reduced to the minimum to highlight the WTFs. Lines from 60 are just to give you something to execute when you want to try running this.