CI-Merchant中的引用ID是事务ID吗？

Question

Is the reference id in the CI-Merchant is the transaction Id ?

object(Merchant_paypal_api_response)#27 (8) { 
    ["_status":protected]=> string(8) "complete" 
    ["_message":protected]=> NULL 
    ["_reference":protected]=> string(17) "0P644392ST628091U" 
    ["_data":protected]=> NULL 
    ["_redirect_url":protected]=> NULL 
    ["_redirect_method":protected]=> string(3) "GET" 
    ["_redirect_message":protected]=> NULL 
    ["_redirect_data":protected]=> NULL 
}

I used this in the call back:

$params = array( 
   'item_name'   => $this->session->userdata('item_name'),  
   'item_number' => $this->session->userdata('item_number'),              
   'amount'      => $this->session->userdata('amount'),    
   'currency'    => 'USD'                 
);

$response=$this->merchant->purchase_return($params);

var_dump($response);  

I want the transaction id.