What is the difference between a session and a cookie in PHP?

In PHP, both sessions and cookies can be used to store data that is unique to a user across multiple requests. However, there are some differences between the two:

1. Data storage: Session data is stored on the server, while cookie data is stored on the client’s computer.
2. Security: Session data is generally more secure than cookie data because it is stored on the server and cannot be easily modified by the user. Cookies, on the other hand, can be easily modified or deleted by the user, so they are less secure.
3. Size limitations: Cookies have a size limit of 4KB, while session data can be much larger.
4. Expiration: Cookies can have an expiration time, after which they are automatically deleted from the client’s computer. Sessions typically expire when the user closes their web browser or after a period of inactivity.
5. Implementation: Sessions are implemented using the session_start() and $_SESSION superglobal array, while cookies are implemented using the setcookie() function and the $_COOKIE superglobal array.

In general, sessions are more secure and flexible than cookies, but they require more server resources and are less portable. Cookies are simpler to use and can be easily implemented on the client side, but they are less secure and have size limitations.