Server Side Includes (SSI) are a simple component system that, when enabled,
instructs the web server to parse the html components in a top-to-bottom
fashion as they are served. The parser scans for tags that look a lot like
HTML comments. In fact, if you have SSI tags embedded in the page without
the server side parsing enabled, your HTML will be served with a bunch of
HTML comments visible when you "View Source." However, when the parser is
enabled, each SSI is replaced with something else such as the contents of
another file or virtual content, a variable value or a formatted date.
It's unlikely that we'll ever see a SAMS Press book titled "Teach Yourself
SSI's in 24 Hours" -- there's just not that much to it. And yet questions
about the capabilities of SSI's arise from time to time. Thus, I've prepared
this mini-tutorial.