The Difference Between PHP Session And Cookies

Sessions and cookies in PHP are both ways to store information about a user between requests, but they have some significant differences.

Cookies:

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.
Cookies are stored on the client's machine, which means they can be manipulated by the client.
The size of a cookie is limited (usually 4KB), so you can't store a lot of information in a single cookie.
Cookies can be set to persist for a long time, until a specific date, or until the user closes their browser. This makes them useful for things like remembering if a user is logged in or their preferences on a site.
Cookies can also be used to track user behavior across different sites (if the sites all belong to the same entity or if they share third-party tracking cookies).

Sessions:

Session data is stored on the server. The server sends the client a cookie with a unique session ID, and the server uses this ID to retrieve the session data. This makes sessions more secure than cookies, because the data isn't exposed to the client.
Session data can be of any size, because it's stored on the server. The only limit is the storage capacity of the server.
Session data is automatically deleted when the session is closed, which usually happens when the user closes their browser. PHP sessions also have a time limit, which is set in the PHP configuration.
Sessions are good for sensitive information, like authentication details, as they are more secure than cookies.

In summary, sessions and cookies are both used for maintaining state between different HTTP requests in a PHP application, but they work in different ways and have different advantages and limitations. Cookies are stored on the client's machine and have size limitations, while sessions are stored on the server and deleted when the session is closed.

How to set and retrieve data with sessions and cookies in PHP:
- Set data with $_SESSION for sessions and setcookie() for cookies. Retrieve data using $_SESSION and $_COOKIE respectively.
```
// Using sessions
session_start();
$_SESSION['username'] = 'john_doe';

// Using cookies
setcookie('username', 'john_doe', time() + 3600, '/');
```