How can I get the value of a variable from the previous page to the current page?

I'm currently creating a login system in PHP. I have 3 files; login.php, checklogin.php, and main.php.
Flow: login.php --- checklogin.php --- main.php

I have a variable inside checklogin.php named as $id.
I assigned value to it:
$id=$_POST['id_number']; //id_number is the name of the textfield in login.php

Now, I want to assign the value of $id from checklogin.php to variable $id_num from main.php and print the value.