PHP Example: Creating an Invoice

<?php

declare(strict_types=1);

$merchantId = getenv('COINSSEND_MERCHANT_ID');
$apiKey = getenv('COINSSEND_API_KEY');

$payload = [
    'order_id' => 'order_'.time(),
    'amount' => '100.50',
    'is_customer_fee' => false,
    'is_customer_network_fee' => false,
    'allow_card_payments' => true,
    'success_url' => 'https://merchant.example/payment-success',
    'cancel_url' => 'https://merchant.example/payment-cancelled',
];

// PHP default json_encode flags are the API's canonical request-body format.
$body = json_encode($payload, JSON_THROW_ON_ERROR);
$sign = md5(base64_encode($body).$apiKey);

$handle = curl_init('https://api.coinssend.com/v1/invoices');
curl_setopt_array($handle, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => $body,
    CURLOPT_HTTPHEADER => [
        'Content-Type: application/json',
        'Merchant: '.$merchantId,
        'Sign: '.$sign,
    ],
]);

$rawResponse = curl_exec($handle);
if ($rawResponse === false) {
    throw new RuntimeException(curl_error($handle));
}

$httpStatus = curl_getinfo($handle, CURLINFO_HTTP_CODE);
curl_close($handle);
$response = json_decode($rawResponse, true, flags: JSON_THROW_ON_ERROR);

if ($httpStatus < 200 || $httpStatus >= 300) {
    throw new RuntimeException($response['error'] ?? $response['message'] ?? "HTTP {$httpStatus}");
}

printf("Invoice %s: %s\n", $response['data']['code'], $response['data']['url']);

Keep the API key server-side. Persist the returned invoice identifiers. A duplicate order_id returns 409 and does not replay the original response, so do not blindly retry an unknown outcome with a new order ID.