A cookie is often used to identify a user. A cookie is a small file that the server embeds on the user's computer.Each time the same computer requests a page with a browser, it will send the cookie too.
A cookie is created with the setcookie() function.
setcookie(name, value, expire, path, domain, secure, httponly);
see more http://tech.queryhome.com/63029/php-session-and-php-cookies