<?php
class BillingServiceFactory
{
    public static function create(User $user): BillingService
    {
        if ($user->isDemo()) {
            return new DemoBillingService();
        } elseif ($user->hasParam1()) {
            return new Api1BillingService($apiV1Url, $apiV1Key);
        } elseif ($user->hasParam2()) {
            return new Api2BillingService($apiV2Url, $apiV1Key);
        } elseif ($user->hasParam3()) {
            return new Api3BillingService($apiV3Url, $apiV1Key);
        }
        return null;
    }
}

class Api1BillingService {
   public function __construct(string $url, string $privateKey) {
        $this->url = $url;
        $this->privateKey = $privateKey;
   }
}