With this SOAP interface (or an equivalent http/https interface) you can seamlessly integrate the IBAN calculator into your business processes.
You can find the interface, as well as its documentation, under this URL:
The WSDL document is here:
For every version of the interface (production/test/development), you can read the corresponding version of the documentation by visiting the SOAP URL with your browser, for example: https://ssl.ibanrechner.de/soap/.
Below you will find the documentation for the current production version:
Here you can find a more complete example which also involves a form.
Here is a Perl Client as a starting point.
Of course, you are not limited to PHP or Perl for your client - you should also be able to use C, C++, Java, Ada, Microsoft .NET, Classic ASP (with the SOAP Toolkit), Python, Delphi, 4th Dimension, and many other programming languages.
All you need to get started is to log in/register here.