Thanks for your suggestion, but it does not work in this case because "$amount" is not filled with the cookie variable. Just tried two ways:
$amount = $_COOKIE["amount"];
$amount = $_COOKIE['amount'];
To be sure the the PHP-script is okay I've tried entering 5678:
$partner_id = '123456';
$amount = '5678';
$description = 'Your order';
With above code the script seems okay. Do you have another suggestion entering the cookie value in $amount?