PHP v2 recurly upgrade subscription add_on

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$subscription = Recurly_Subscription::get('17caaca1716f33572edc8146e0aaefde');
$add_on = new Recurly_SubscriptionAddOn(); 
$add_on->quantity = 2;  
$add_on->add_on_code = 'extraip'; 
$subscription->subscription_add_ons[] = $add_on;
$subscription->updateImmediately();