A dummy plugin for any site that runs PHP.
Here is an extract from the PHP manual
PHP transparently supports HTTP cookies. Cookies are a mechanism for storing data in the remote browser and thus tracking or identifying return users. You can set cookies using the setcookie() or setrawcookie() function. Cookies are part of the HTTP header, so setcookie() must be called before any output is sent to the browser. This is the same limitation that header() has. You can use the output buffering functions to delay the script output until you have decided whether or not to set any cookies or send any headers.
Any cookies sent to you from the client will automatically be included into a $_COOKIE auto-global array if variables_order contains “C”. If you wish to assign multiple values to a single cookie, just add [] to the cookie name.