What are the best practices for integrating PayPal IPN and API in PHP for instant payment confirmation?

When integrating PayPal IPN and API in PHP for instant payment confirmation, it is important to set up a listener to receive and process IPN messages from PayPal. This listener should verify the IPN message with PayPal to prevent fraud and ensure the payment is legitimate. Additionally, using the PayPal API can provide real-time payment information and allow for additional functionality such as refunds or order management. 

// Sample PHP code for integrating PayPal IPN and API

// Set up IPN listener to receive and process IPN messages
$listener = new \PayPal\IPN\Listener();
$verified = $listener->verifyIPN();

if ($verified) {
    // Process the IPN message and update payment status
    $payment_status = $_POST['payment_status'];
    
    if ($payment_status == 'Completed') {
        // Payment is successful, update database or trigger order fulfillment
    }
    
    // Use PayPal API to fetch additional payment details if needed
    $api = new \PayPal\Api\Payment();
    $payment_id = $_POST['txn_id'];
    $payment = $api->getPaymentDetails($payment_id);
    
    // Additional functionality like refunds or order management can be implemented here
} else {
    // IPN message verification failed, log error or take appropriate action
}

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