If you are at all interested in hosting a simple (or complex) website or doing some experimentation from your older Mac, I do recommend grabbing the incredibly useful XAMPP to make the process a little easier.
XAMPP is kind of a self-contained install of the Apache httpd server with PHP and CGI turned on, a MYSQL server, an FTP server, and some other tools and documentation. It’s cool, because it comes with a little tiny app that lets you turn off the services as you see fit. You can run it when you want to try out different configurations or different websites.
For Leopard, the basic Apache httpd server isn’t that older than the last available XAMPP package for G5 machines. My base Leopard httpd server is 2.2.7 while the one included in the XAMPP package is 2.2.14. Still, that includes some bug fixes as well as newer versions of PHP and MySQL. If you are going to run old software, might as well run the last version of it you can find.
Note: I do believe, using MacPorts or TigerBrew or something, you can get a more recent compiled version of httpd (2.4 series) – maybe even doing it on your own. You are likely better in that scenario of turning to Linux if you really care about security and want things fully patched (and maintained).
For now though, having access to an older version of XAMPP is still kind of fun to mess around with. Grab it here.
Another note: It is generally not recommended to copy over the httpd server in Leopard with a newer one. Now, it should technically work – we are talking about bug and security fixes from 2.2.7 to 2.2.14, for instance. However, it is probably best to be safe, keep backups, and just use an alternative like XAMPP which messes with system internals as little as possible.