index.php

<?php
$url = "https://api.paystack.co/refund/retry_with_customer_details/{id}";

$fields = [
  'transaction' => 1641,
  "refund_account_details" => [
    "currency" => "NGN",
    "account_number" => "1234567890",
    "bank_id" => "9"
  ]
];

$fields_string = http_build_query($fields);

//open connection
$ch = curl_init();

//set the url, number of POST vars, POST data
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields_string);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
  "Authorization: Bearer SECRET_KEY",
  "Cache-Control: no-cache",
));

//So that curl_exec returns the contents of the cURL; rather than echoing it
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

//execute post
$result = curl_exec($ch);
echo $result;