Which license for self-contained, external extensions?

Hello PHP developers, Thanks a lot for PHP 4! I have created a self-contained, external PHP 4 extension which enables PHP 4 to use Cashcow-functions. Cashcow is a LGPL'ed piece of software which Danish e-commerce'ers may use to communicate with the Danish credit card payment authority ("PBS"). The extension is at http://download.cashcow.dk/source/php/php4/ Now, which license should I use for the extension? The library which I speak to from the PHP extension is LGPL'ed. PHP 4 is ... hmm ... under the "PHP License" _and_ the "Zend License". So there are three different licenses involved! My current choice is to release my extension under the GPL license, but is that valid? - I mean, I use PHP code (phpize, header-files, etc.) to create the extension. By the way: I have also created some PHP 4 RPMs of general interest: http://fsr.ku.dk/people/troels/rpms/php/ -- Greetings from Troels Arvin, Copenhagen, Denmark dipCard.com