A server-side include is a variable value (for example, a file "Last modified" date) that a server can include in an HTML file before it sends it to the requestor. If you're creating a Web page, you can insert an include statement in the HTML file that looks like this:
and the server will obtain the last-modified date for the file and insert it before the HTML file is sent to requestors.
LAST_MODIFIED is one of several environment variables that an operating system can keep track of and that can be accessible to a server program. The server administrator can make these environment variables usable when the system is set up.
A Web file that contains server-side include statements (such as the "echo" statement above) is usually defined by the administrator to be a file with an "." suffix. You can think of a server-side include as a limited form of common gateway interface application. In fact, the CGI is not used. The server simply searches the server-side include file for CGI environment variables, and inserts the variable information in the places in the file where the "include" statements have been inserted.
When creating a Web site, a good idea is to ask your server administrator which environment variables can be used and whether the administrator can arrange to set the server up so that these can be handled. The administrator may be able to help you add the appropriate "include" statements to your HTML file.