Differentiate expired and unpaid-but-still-valid invoices

client.php

@@ -45,13 +45,18 @@ public function fetch($invoice_id) {
    *
    * @param string $invoice_id
    * @param int $timeout the timeout in seconds
-   * @return object|bool the paid invoice if paid, false otherwise.
+   * @return object|bool|null the paid invoice if paid, false if the invoice expired, or null if the timeout is reached.
    */
   public function wait($invoice_id, $timeout) {
     $res = $this->api->get('/invoice/' . urlencode($invoice_id) . '/wait?timeout=' . (int)$timeout);
 
+    // 402 Payment Required: timeout reached without payment, invoice is still payable
     if ($res->info->http_code === 402)
+      return null;
+    // 410 Gone: invoice expired and can not longer be paid
+    else if ($res->info->http_code === 410)
       return false;
+    // 200 OK: invoice is paid, returns the updated invoice
     else if ($res->info->http_code === 200)
       return $res->decode_response();
     else