What role does the session_start function play in maintaining session data integrity in PHP applications?

The session_start function in PHP is essential for maintaining session data integrity in PHP applications. It initializes a session or resumes the current one based on a session identifier passed via a GET or POST request, or a cookie. Without calling session_start, session data cannot be accessed or stored across multiple pages.

<?php
session_start();

// Access or store session data here
?>