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

Computer is an electronic data processing device capable of taking an input data, processing them and displaying the processed result at a very high speed under the control of set of instructions. The four features of computer are :

• Speed
• Accuracy 
• Versatility 
•  Diligence 

Data is defined as the raw facts or symbols which are collected from various sources and need to be processed. 

Information is defined as the desired or meaning full result obtained after processing the data.

The capabilities of computer are listed below :

• Computers operate at a very high speed, in fact with the speed of light.
• Computers operate nearly 100% accurately.
• Computers is capable of functioning automatically.
• Computer is a  versatile device.
• Computer stores large volume of data and information. 

Computer is called diligence machine because it can perform the task repeatedly without loosing its speed and accuracy for long time. 

The circuitries of computers are designed on the basis of electrical signals and data travel them as the speed of current, therefore the computer is able to perform any task at the high speed.