HTTP/cookie

Overview
In HTTP, a cookie is a piece of information sent to the web browser for the browser to send back to the web server on subsequent requests. Cookies are most commonly used to identify of a particular user across multiple accesses, but they can also be used to store preferences and other persistent-state information without specifically tracking a given user.

Programming
In PHP, the $_COOKIE[] built-in global array and setcookie function are the most common means of retrieving and setting cookies, but PHP does have an extensive selection of cookie-management functionality.