Missing Apple App Secret in your code

You need to send request with "Apple App Secret" to validate receipt otherwise it returns 500 - Internal server error.

So, I made following changes in your code where you need to past your apple app secret 

private function encodeRequest() {
        return json_encode(array('receipt-data' => $this->getReceipt(), 'password' => 'APPLE_APP_SECRET'));
    }
