Getting transaction details using PayPal REST API (v2) not working - php

Question

I want to get the full details of a PayPal transaction by its ID using the PayPal REST API.
I tried to do it as the following:
<?php
include("config.php");
// Reference: https://developer.paypal.com/docs/api/get-an-access-token-curl/
$ch = curl_init("https://api.paypal.com/v1/oauth2/token");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Accept: application/json",
"Accept-Language: en_US"
));
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, "grant_type=client_credentials");
curl_setopt($ch, CURLOPT_USERPWD, $restapp['client_id'] . ':' . $restapp['secret']);
$result = curl_exec($ch);
curl_close($ch);
$access_token = json_decode($result, true)['access_token'];
// Reference: https://developer.paypal.com/docs/checkout/reference/server-integration/get-transaction/#on-the-server
$ch = curl_init("https://api.paypal.com/v2/checkout/orders/<transaction_id>");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Content-Type: application/json",
"Accept: application/json",
"Authorization: Bearer " . $access_token
));
$result = curl_exec($ch);
curl_close($ch);
echo $result;
?>
But this always returns error INVALID_RESOURCE_ID. Please note that I am using the client ID and client secret of my live application, and the transaction ID that I use is a live transaction ID too.
After doing some searches, I've found that https://api.paypal.com/v1/payments/orders/<transaction_id> works instead of https://api.paypal.com/v2/checkout/orders/<transaction_id> .. but v1 doesn't return as much information as v2
What am I doing wrong on the v2 endpoint?
Kind regards.