Users who aren't interested in the details of versions may skip this section: it is intended primarily for System Administrators.
PHP is available both in versions 3 and 4 right now: it seems that the world has been rather slow to adapt version 4, perhaps because v3 works so well. I have continued use version 3 for several reasons, the largest of which is that my main web server uses RPM packages, and version 4 isn't available for its architecture. I have no compelling reason to compile and install v4 myself, as v3 runs exceptionally well. There have been changes to the language between versions, but the main change has been a complete rewrite of the server code to speed it up. Version 4 is supposed to be much faster: if I were running a high-end commercial server that was running near capacity, I would perhaps be more interested in being an early adapter.
Version 4 can be found at http://www.php.net/. There is also excellent documentation available here. Most of my servers are RedHat based, so I used RPM packages found at http://rufus.w3.org/linux/RPM/. There was also a version for the Cobalt Qube II (which I believe also runs on the Qube and the RAQ) available at Cobalt's FTP site, although I could no longer find it in July 2000. This was version 3.07, and that's what I'm working from. Considering that this presentation is on basics, the version shouldn't be important, but you have it just in case.