PHP is one of the scripting language. In which session handling is one of the key thing mostly using in web application.
Suppose you build a website and allow to login everyone in website, You need to track user every step until they log out our system, Its called Session tracking.
Now the question is why we need to track the session, answer is very simple .HTTP state less protocol, When you refresh you page, Its lost every information, So this is the reason we need session handling.
To handle session in php we need to a global variable $_SESSION and session_start()
– Read full more at: Using Database Session Handling in Code PHP