Beta versions are currently available. I'm also working on a book that will cover PHP 4. A recent E-Soft survey indicates that PHP is now the most popular Apache module. I think it is mostly because the barrier of entry to PHP development is low. It is also free and bundled with all the popular free Unix distributions, so a lot of people have access to it.
People tend to try what they have at hand before looking for alternatives.
PHP's success is that once people try it they tend to keep using it. Because it works. Another really important factor to PHP's success has been the dynamic community that has been built up behind the language. There are support Web sites, mailing lists, and user groups around the world, working to help each other with problems common to Web development. People like being part of a community, and given the open-source nature of PHP, this community is driven by personal pride and motivation, not financial benefit.