The following table describes common CGI environment variables the browser creates and passes in the request header:

CGI client variable

Description

HTTP_REFERER

The referring document that linked to or submitted form data.

HTTP_USER_AGENT

The browser that the client is currently using to send the request. Format: software/version library/version.

HTTP_IF_MODIFIED_SINCE

The last time the page was modified. The browser determines whether to set this variable, usually in response to the server having sent the LAST_MODIFIED HTTP header. It can be used to take advantage of browser-side caching.

HTTP_URL The URL path in an encoded format.

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