Get Subscription Transactions API Call after successful Subscription Checkout

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Hello, everyone.

I have an issue with the GET Subscription Transactions request that I hope you guys can help me out with.

I have added a Smart Subscribe Button on my site using PayPal JS SDK. In the onApprove handler, I send the subscription ID and order ID to a backend script so I can create a DB record for the new PayPal user/subscriber and a transaction record for the Trial Fee Payment.

In the backend script (PHP), I am able to successfully do a GET Subscription Details call to the Subscriptions API which I need for the User and Subscriber DB records. But once the backend script tries to do GET Subscription Transactions request for the new Transaction DB record, the API returns an empty JSON {} response. 😓

For debugging, I've tried hard-coding an existing Subscription ID and Order ID into the backend script, and the process works as it should be and I am able to get the subscription transactions. But whenever, it starts using the Subscription ID and Order ID from a successful PayPal Checkout, the request for the subscription transactions becomes empty again. (Which makes me scratch my head every single time...)

For reference, here are my function definitions for these two PayPal API Calls:

GET Subscription

function getPaypalSubscription($subscriptionId, $paypalAccessToken)
{
    $apiUrl = $_ENV['PAYPAL_API_URL'] . $_ENV['PAYPAL_API_VERSION'] . '/billing/subscriptions/' . $subscriptionId;
    $curl = curl_init();
    curl_setopt_array($curl, array(
        CURLOPT_URL => $apiUrl,
        CURLOPT_RETURNTRANSFER => 1,
        CURLOPT_CUSTOMREQUEST => 'GET',
        CURLOPT_HTTPHEADER => array(
            'Authorization: Bearer ' . $paypalAccessToken,
        ),
    ));

    $paypalSubscription = curl_exec($curl);
    curl_close($curl);
    return json_decode($paypalSubscription);
}

GET Subscription Transactions

function getPaypalSubscriptionTransactions($subscriptionId, $paypalAccessToken, $startTime, $endTime)
{
    $apiUrl = $_ENV['PAYPAL_API_URL'] . $_ENV['PAYPAL_API_VERSION'] . '/billing/subscriptions/' . $subscriptionId . '/transactions';
    $queryParams = array(
        'start_time' => $startTime,
        'end_time' => $endTime
    );
    $apiUrl .= '?' . http_build_query($queryParams);
    $curl = curl_init();
    curl_setopt_array($curl, array(
        CURLOPT_URL => $apiUrl,
        CURLOPT_RETURNTRANSFER => 1,
        CURLOPT_CUSTOMREQUEST => 'GET',
        CURLOPT_HTTPHEADER => array(
            'Authorization: Bearer ' . $paypalAccessToken,
        ),
    ));

    $paypalSubscriptionTxns = curl_exec($curl);
    curl_close($curl);
    return json_decode($paypalSubscriptionTxns);
}