Only ever return implementations of an interface as an anonymous class coming from a factory.

anonymous_class_interface_implementation.php

<?php

/**
 * https://twitter.com/Ocramius/status/959405445260726272
 * only ever return implementations of an interface as an
 * anonymous class coming from a factory.
 */

namespace Example;

use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;

require __DIR__ . '/vendor/autoload.php';

interface StringModifier
{
    public function modify(string $name): string;
}


final class UpperCaseStringModifier
{
    private function __construct() {}

    public static function create(LoggerInterface $logger): StringModifier
    {
        return new class ($logger) implements StringModifier {
            private $logger;

            public function __construct(LoggerInterface $logger)
            {
                $this->logger = $logger;
            }

            public function modify(string $name): string
            {
                $this->logger->debug('some log');
                return strtoupper($name);
            }
        };
    }
}


$logger = new NullLogger();
$stringModifier = UpperCaseStringModifier::create($logger);

echo $stringModifier->modify('Ocramius is crazy');