Web FAQ 6.01. What are server-side includes?
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Server-side includes allow a server to include information from files and environmental variables into an HTML document before that document leaves the server. The pages are parsed (or created on the "fly" following a set of rules) by the server based on the server-side includes in the HTML source files. They appear as a comment in the HTML text but cause the server to replace the "comment" with a text string that is the end result of the server-side include command.