How to define Activity return values in PHP

Return values must be serializable to a byte array using the provided DataConverter interface. The default implementation uses a JSON serializer, but an alternative implementation can be easily configured. Thus, you can return both primitive types:

class GreetingActivity implements GreetingActivityInterface
{
    public function composeGreeting(string $greeting, string $name): string
    {
        return $greeting . ' ' . $name;
    }
}

And objects:

class GreetingActivity implements GreetingActivityInterface
{
    public function composeGreeting(string $greeting, string $name): Greeting
    {
        return new Greeting($greeting, $name);
    }
}