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. With PHP, you can both create and retrieve cookie values.  A cookie is created with the setcookie() function.

For detailed information, you can walk through this video:

Here is the source code of Chat Application by another YouTube Creator.
https://www.dropbox.com/s/bslb562b9kt8era/chat.zip

Here is an explanation of the source code— https://youtu.be/XkV9XdHueL8