How can the issue of the variable not being stored in the session be troubleshooted effectively in PHP?

The issue of the variable not being stored in the session in PHP can be effectively troubleshooted by ensuring that the session_start() function is called before trying to store or access session variables. This function initializes a session or resumes the current one. Without calling session_start(), PHP will not be able to access or store session variables.

<?php
// Start the session
session_start();

// Store a variable in the session
$_SESSION['variable_name'] = 'variable_value';

// Access the stored variable
echo $_SESSION['variable_name'];
?>