Authorize.net Integration with CakePHP 3

Hi,
I am trying to integrate authorizenet payment recurrring billing with my cakePHP 3 project. Adding

"require": {      "authorizenet/authorizenet": "~1.8"  }  to the composer I installed the library file in vendor. I couldn't find any namespace declarations on the library files. I created a function on my vendor controller as below
public function authtest(){
        define("AUTHORIZENET_API_LOGIN_ID", "LOGIN");
        define("AUTHORIZENET_TRANSACTION_KEY", "KEY");
        $subscription                          = new AuthorizeNet_Subscription;
        $subscription->name                    = "PHP Monthly Magazine";
        $subscription->intervalLength          = "1";
        $subscription->intervalUnit            = "months";
        $subscription->startDate               = "2014-08-14";
        $subscription->totalOccurrences        = "12";
        $subscription->amount                  = "12.99";
        $subscription->creditCardCardNumber    = "6011000000000012";
        $subscription->creditCardExpirationDate= "2018-10";
        $subscription->creditCardCardCode      = "123";
        $subscription->billToFirstName         = "Rasmus";
        $subscription->billToLastName          = "Doe";

        // Create the subscription.
        $request         = new AuthorizeNetARB;
        $response        = $request->createSubscription($subscription);
        $subscription_id = $response->getSubscriptionId();
        echo $subscription_id;exit;
    }
When I am accessing the function ad dsite.com/vendors/authtest I get the following error.

Fatal Error

Error: Class 'App\Controller\AuthorizeNet_Subscription' not found

Would you please help me to fix this?
Regards,
Jipson

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