Firstly let us know what is session ?

Session variables hold information about one single user, and provide to all pages information in one application.
So, session basicallu store informatiom in a variable and provine in multiple page for a single user How to start a session

session_start();

Note:- session starts Before any HTML tags.

You can see below example of session in php.

<?php

// fix session variables
$_SESSION["favcity"] = "Landon";
$_SESSION["favcountry"] = "UK";
// echo  session variables  
echo "Session variables will come". ".<br>";

//now here call session variables that were set on previous page
echo "Favorite city is " . $_SESSION["favcity"] . ".<br>";
echo "Favorite country is " . $_SESSION["favcountry"] . ".";

?>

Output will come:
Session variables will come.
Favorite city is Landon.
Favorite country is UK.