How does WP Greet Box work?

I have noticed some blogs using what looks like a wordpress plugin to detect if the website visitor came from google se开发者_如何学Pythonarch. how is this done?

http://omninoggin.com/projects/wordpress-plugins/wp-greet-box-wordpress-plugin/

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The plugin checks the $_SERVER['HTTP_REFERRER'] value (since it is written in PHP) which holds the referrer of the loaded page, then parses it to check what site the user came from to welcome them accordingly.