As one of the co-authors of the Redbook you are pointing to, the reason for not mentioning PHP is quite simple. PHP is not a Domino language or not even a new thing at all. PHP has been around for many years, and the integration with Domion and PHP has been possible for years. The new thing about integrating PHP and Domino in ND6 is more like a systemadministration setup. There is nothing a application developer needs to do, really. Ok - it might have been mentioned, but again - it should rather be mentioned in a systemadministration Redbook for Domino 6.
Regards,
Rune Carlsen
Founder Dominozone.net
http://www.dominozone.net