gistfile1.php

<?php

interface Validator {
    function validate($value, Error &$error = null);
}
class ValidatorA implements Validator {}
class ValidatorB implements Validator {}

class SimpleObject {
    function foo($baz) {
        $error = null;
        foreach($this->validatorList AS $validator) {
            $validator->validate($baz, $error);
        }

        var_dump($error);
        // type Error
    }
}