Server Side Includes (SSI) provides a simple way to embed executable code into a web page. The advantages of SSI are that it is simple to use and easy to learn, and sometimes simplicity is its own reward. The main drawback of SSI is that because of its simplicity, it is limited in what it can do. 
SSI is not CGI (see Chapter 7)—that is, it is not a program that is executed to generate HTML text, so one needn’t be a Perl hacker to use it. Rather, SSI is a collection of directives that can be embedded in an HTML file and processed by Apache. A directive has this form:
<!--#directive param1="value1" param2="value2"... -->
which is simply an HTML comment with a “#” escape to ...