IMCA - Web Development Using PHP

The main difference between sessions and cookies in PHP is where the data is stored . 

• Cookies are stored on the user's browser , and they can see and sometimes change them. Cookies are used to keep track of small pieces of information like a username or a theme preference. They can be set to last for a certain time, even after the browser is closed.

• Sessions , on the other hand, are stored on the server , not on the user's computer. Only a unique ID (called a session ID) is sent to the browser, usually through a cookie. This makes sessions more secure than cookies because users can’t directly access the data stored in them. 

In short:
Use cookies for non-sensitive data that you don’t mind being stored on the user’s computer.
Use sessions for sensitive or temporary data that should stay secure and only accessible on the server.