PHP Becoming Mainstream?

Tim O’Reilly says that the PHP & MySQL For Dummies is now the top selling book for that language, so therefore PHP is becoming mainstream (via John Gruber). I’m confused. I thought PHP 3 was popular, with many hosts including support for it. PHP 4 was definitely mainstream, by just about any definition I can think of. Now PHP 5 is already almost three years old. I guess I don’t know what “mainstream” means.